diff --git a/Makefile b/Makefile
index b5f37dd..e4d646d 100644
--- a/Makefile
+++ b/Makefile
@@ -1,6 +1,6 @@
 test: check
 	poetry run maturin develop --release
-	poetry run pytest -v .
+	poetry run pytest .
 
 check:
 	cargo fmt --check
diff --git a/haem.pyi b/haem.pyi
index f92f2f7..0a5a272 100644
--- a/haem.pyi
+++ b/haem.pyi
@@ -56,9 +56,13 @@ class DNABase:
         ...
     def __invert__(self) -> DNABase:
         """See `DNABase.complement`."""
-    def __add__(self, other: typing.Union[DNABase, DNASequence]) -> DNASequence:
-        """Create a DNA sequence from the concatenation of this base and either
-        another base or a DNA sequence."""
+    def __add__(self, other: typing.Union[DNABase, DNASequence, str]) -> DNASequence:
+        """Create a new sequence consisting of this base followed by the given
+        sequence member(s)."""
+        ...
+    def __radd__(self, other: typing.Union[DNABase, DNASequence, str]) -> DNASequence:
+        """Create a new sequence consisting of the given sequence member(s)
+        followed by this base."""
         ...
 
 class RNABase:
@@ -111,9 +115,13 @@ class RNABase:
         ...
     def __invert__(self) -> RNABase:
         """See `RNABase.complement`."""
-    def __add__(self, other: typing.Union[RNABase, RNASequence]) -> RNASequence:
-        """Create a RNA sequence from the concatenation of this base and either
-        another base or a RNA sequence."""
+    def __add__(self, other: typing.Union[RNABase, RNASequence, str]) -> RNASequence:
+        """Create a new sequence consisting of this base followed by the given
+        sequence member(s)."""
+        ...
+    def __radd__(self, other: typing.Union[RNABase, RNASequence, str]) -> RNASequence:
+        """Create a new sequence consisting of the given sequence member(s)
+        followed by this base."""
         ...
 
 class AminoAcid:
@@ -186,10 +194,16 @@ class AminoAcid:
         """Always true."""
         ...
     def __add__(
-        self, other: typing.Union[AminoAcid, AminoAcidSequence]
+        self, other: typing.Union[AminoAcid, AminoAcidSequence, str]
+    ) -> AminoAcidSequence:
+        """Create a new sequence consisting of this amino acid followed by the
+        given sequence member(s)."""
+        ...
+    def __radd__(
+        self, other: typing.Union[AminoAcid, AminoAcidSequence, str]
     ) -> AminoAcidSequence:
-        """Create an amino acid sequence from the concatenation of this amino
-        acid and either another amino acid or an amino acid sequence."""
+        """Create a new sequence consisting of the given sequence member(s)
+        followed by this amino acid."""
         ...
 
 class DNASequence:
@@ -232,9 +246,13 @@ class DNASequence:
     def __bool__(self) -> bool:
         """Casting to bool is False for empty sequences and True otherwise."""
         ...
-    def __add__(self, other: typing.Union[DNABase, DNASequence]) -> DNASequence:
-        """Create a DNA sequence from the concatenation of this sequence and
-        either a DNABase or another DNASequence."""
+    def __add__(self, other: typing.Union[DNABase, DNASequence, str]) -> DNASequence:
+        """Create a new sequence consisting of this sequence followed by the
+        given sequence member(s)."""
+    def __radd__(self, other: typing.Union[DNABase, DNASequence, str]) -> DNASequence:
+        """Create a new sequence consisting of the given sequence member(s)
+        followed by this sequence."""
+        ...
     def __contains__(self, item: typing.Union[DNABase, DNASequence]) -> bool:
         """Return true if the given DNABase or DNASequence is contained within
         this sequence."""
@@ -288,9 +306,13 @@ class RNASequence:
     def __bool__(self) -> bool:
         """Casting to bool is False for empty sequences and True otherwise."""
         ...
-    def __add__(self, other: typing.Union[RNABase, RNASequence]) -> RNASequence:
-        """Create a RNA sequence from the concatenation of this sequence and
-        either a RNABase or another RNASequence."""
+    def __add__(self, other: typing.Union[RNABase, RNASequence, str]) -> RNASequence:
+        """Create a new sequence consisting of this sequence followed by the
+        given sequence member(s)."""
+    def __radd__(self, other: typing.Union[RNABase, RNASequence, str]) -> RNASequence:
+        """Create a new sequence consisting of the given sequence member(s)
+        followed by this sequence."""
+        ...
     def __contains__(self, item: typing.Union[RNABase, RNASequence]) -> bool:
         """Return true if the given RNABase or RNASequence is contained within
         this sequence."""
@@ -337,10 +359,16 @@ class AminoAcidSequence:
         """Casting to bool is False for empty sequences and True otherwise."""
         ...
     def __add__(
-        self, other: typing.Union[AminoAcid, AminoAcidSequence]
+        self, other: typing.Union[AminoAcid, AminoAcidSequence, str]
     ) -> AminoAcidSequence:
-        """Create an amino acid sequence from the concatenation of this
-        sequence and either an AminoAcid or another AminoAcidSequence."""
+        """Create a new sequence consisting of this sequence followed by the
+        given sequence member(s)."""
+    def __radd__(
+        self, other: typing.Union[AminoAcidSequence, AminoAcid, str]
+    ) -> AminoAcidSequence:
+        """Create a new sequence consisting of the given sequence member(s)
+        followed by this sequence."""
+        ...
     def __contains__(self, item: typing.Union[AminoAcid, AminoAcidSequence]) -> bool:
         """Return true if the given AminoAcid or AminoAcidSequence is contained
         within this sequence."""
diff --git a/src/aminoacid.rs b/src/aminoacid.rs
index 295963f..8341400 100644
--- a/src/aminoacid.rs
+++ b/src/aminoacid.rs
@@ -1,6 +1,7 @@
 use crate::aminoacidsequence::AminoAcidSequence;
-use crate::member::{Member, MemberOrMembers};
+use crate::member::Member;
 use crate::rnabase::RNABase;
+use crate::utils::AddInput;
 use pyo3::class::basic::CompareOp;
 use pyo3::create_exception;
 use pyo3::prelude::*;
@@ -195,10 +196,16 @@ impl AminoAcid {
         true
     }
 
-    fn __add__(&self, other: MemberOrMembers<Self>) -> AminoAcidSequence {
-        AminoAcidSequence {
-            amino_acids: self.add(other),
-        }
+    fn __add__(&self, other: AddInput<Self>) -> PyResult<AminoAcidSequence> {
+        Ok(AminoAcidSequence {
+            amino_acids: self.add(other, false)?,
+        })
+    }
+
+    fn __radd__(&self, other: AddInput<Self>) -> PyResult<AminoAcidSequence> {
+        Ok(AminoAcidSequence {
+            amino_acids: self.add(other, true)?,
+        })
     }
 }
 
diff --git a/src/aminoacidsequence.rs b/src/aminoacidsequence.rs
index fc88f54..4acaf95 100644
--- a/src/aminoacidsequence.rs
+++ b/src/aminoacidsequence.rs
@@ -1,7 +1,7 @@
 use crate::aminoacid::AminoAcid;
 use crate::member::{MemberOrCode, MemberOrMembers};
 use crate::sequence::{Sequence, SequenceInput};
-use crate::utils::IntOrSlice;
+use crate::utils::{AddInput, IntOrSlice};
 use pyo3::class::basic::CompareOp;
 use pyo3::prelude::*;
 
@@ -41,10 +41,16 @@ impl AminoAcidSequence {
         self.bool()
     }
 
-    fn __add__(&self, other: MemberOrMembers<AminoAcid>) -> Self {
-        Self {
-            amino_acids: self.add(other),
-        }
+    fn __add__(&self, other: AddInput<AminoAcid>) -> PyResult<Self> {
+        Ok(Self {
+            amino_acids: self.add(other, false)?,
+        })
+    }
+
+    fn __radd__(&self, other: AddInput<AminoAcid>) -> PyResult<Self> {
+        Ok(Self {
+            amino_acids: self.add(other, true)?,
+        })
     }
 
     fn __contains__(&self, amino_acid_or_seq: MemberOrMembers<AminoAcid>) -> PyResult<bool> {
diff --git a/src/dnabase.rs b/src/dnabase.rs
index 67b0aba..4535753 100644
--- a/src/dnabase.rs
+++ b/src/dnabase.rs
@@ -1,6 +1,7 @@
 use crate::dnasequence::DNASequence;
-use crate::member::{Member, MemberOrMembers};
+use crate::member::Member;
 use crate::rnabase::RNABase;
+use crate::utils::AddInput;
 use pyo3::class::basic::CompareOp;
 use pyo3::prelude::*;
 use std::fmt;
@@ -107,10 +108,16 @@ impl DNABase {
         self.get_complement()
     }
 
-    fn __add__(&self, other: MemberOrMembers<Self>) -> DNASequence {
-        DNASequence {
-            bases: self.add(other),
-        }
+    fn __add__(&self, other: AddInput<Self>) -> PyResult<DNASequence> {
+        Ok(DNASequence {
+            bases: self.add(other, false)?,
+        })
+    }
+
+    fn __radd__(&self, other: AddInput<Self>) -> PyResult<DNASequence> {
+        Ok(DNASequence {
+            bases: self.add(other, true)?,
+        })
     }
 
     fn __str__(&self) -> String {
diff --git a/src/dnasequence.rs b/src/dnasequence.rs
index 345cc85..daeddec 100644
--- a/src/dnasequence.rs
+++ b/src/dnasequence.rs
@@ -3,7 +3,7 @@ use crate::member::{MemberOrCode, MemberOrMembers};
 use crate::rnabase::RNABase;
 use crate::rnasequence::RNASequence;
 use crate::sequence::{Sequence, SequenceInput};
-use crate::utils::IntOrSlice;
+use crate::utils::{AddInput, IntOrSlice};
 use pyo3::class::basic::CompareOp;
 use pyo3::prelude::*;
 use rayon::prelude::*;
@@ -61,10 +61,16 @@ impl DNASequence {
         self.bool()
     }
 
-    fn __add__(&self, other: MemberOrMembers<DNABase>) -> Self {
-        Self {
-            bases: self.add(other),
-        }
+    fn __add__(&self, other: AddInput<DNABase>) -> PyResult<Self> {
+        Ok(Self {
+            bases: self.add(other, false)?,
+        })
+    }
+
+    fn __radd__(&self, other: AddInput<DNABase>) -> PyResult<Self> {
+        Ok(Self {
+            bases: self.add(other, true)?,
+        })
     }
 
     fn __len__(&self) -> usize {
diff --git a/src/member.rs b/src/member.rs
index cca77e9..64d4f39 100644
--- a/src/member.rs
+++ b/src/member.rs
@@ -1,4 +1,4 @@
-use crate::utils::Wrapper;
+use crate::utils::{AddInput, Wrapper};
 use pyo3::class::basic::CompareOp;
 use pyo3::prelude::*;
 use pyo3::pyclass::PyClass;
@@ -6,22 +6,49 @@ use pyo3::types::PyIterator;
 use rayon::prelude::*;
 
 pub trait Member<T> {
-    fn add(&self, other: MemberOrMembers<T>) -> Vec<T>;
+    fn add(&self, other: AddInput<T>, swap: bool) -> PyResult<Vec<T>>;
     fn richcmp(&self, other: &Self, op: CompareOp, py: Python<'_>) -> PyObject;
 }
 
-impl<T: Clone + PartialEq> Member<T> for T {
-    fn add(&self, other: MemberOrMembers<T>) -> Vec<T> {
-        match other {
-            MemberOrMembers::Member(member) => {
-                vec![self.clone(), member]
+impl<T: Clone + PartialEq> Member<T> for T
+where
+    T: TryFrom<char, Error = PyErr> + Send + Clone,
+{
+    fn add(&self, other: AddInput<T>, swap: bool) -> PyResult<Vec<T>> {
+        Ok(match other {
+            AddInput::Member(member) => match swap {
+                true => vec![member, self.clone()],
+                false => vec![self.clone(), member],
+            },
+            AddInput::Members(members) => {
+                let mut sequence = Vec::with_capacity(members.len() + 1);
+                match swap {
+                    false => {
+                        sequence.push(self.clone());
+                        sequence.extend(members);
+                    }
+                    true => {
+                        sequence.extend(members);
+                        sequence.push(self.clone());
+                    }
+                }
+                sequence
             }
-            MemberOrMembers::Sequence(sequence) => {
-                let mut sequence = sequence;
-                sequence.insert(0, self.clone());
+            AddInput::Codes(codes) => {
+                let mut sequence = Vec::with_capacity(codes.len() + 1);
+                match swap {
+                    false => {
+                        sequence.push(self.clone());
+                        sequence.extend(Wrapper::try_from(codes)?.into_inner());
+                    }
+                    true => {
+                        sequence.extend(Wrapper::try_from(codes)?.into_inner());
+                        sequence.push(self.clone());
+                    }
+                }
                 sequence
             }
-        }
+        })
     }
 
     fn richcmp(&self, other: &Self, op: CompareOp, py: Python<'_>) -> PyObject {
@@ -59,8 +86,8 @@ where
     fn try_from(codes: Vec<char>) -> PyResult<Self> {
         Ok(Wrapper(
             codes
-                .par_iter()
-                .map(|c| T::try_from(*c))
+                .into_par_iter()
+                .map(T::try_from)
                 .collect::<PyResult<_>>()?,
         ))
     }
diff --git a/src/rnabase.rs b/src/rnabase.rs
index 06eef8c..601c443 100644
--- a/src/rnabase.rs
+++ b/src/rnabase.rs
@@ -1,6 +1,7 @@
 use crate::dnabase::DNABase;
-use crate::member::{Member, MemberOrMembers};
+use crate::member::Member;
 use crate::rnasequence::RNASequence;
+use crate::utils::AddInput;
 use pyo3::class::basic::CompareOp;
 use pyo3::prelude::*;
 use std::fmt;
@@ -107,10 +108,16 @@ impl RNABase {
         self.get_complement()
     }
 
-    fn __add__(&self, other: MemberOrMembers<Self>) -> RNASequence {
-        RNASequence {
-            bases: self.add(other),
-        }
+    fn __add__(&self, other: AddInput<Self>) -> PyResult<RNASequence> {
+        Ok(RNASequence {
+            bases: self.add(other, false)?,
+        })
+    }
+
+    fn __radd__(&self, other: AddInput<Self>) -> PyResult<RNASequence> {
+        Ok(RNASequence {
+            bases: self.add(other, true)?,
+        })
     }
 
     fn __str__(&self) -> String {
diff --git a/src/rnasequence.rs b/src/rnasequence.rs
index 58c8917..212f148 100644
--- a/src/rnasequence.rs
+++ b/src/rnasequence.rs
@@ -3,7 +3,7 @@ use crate::dnasequence::DNASequence;
 use crate::member::{MemberOrCode, MemberOrMembers};
 use crate::rnabase::RNABase;
 use crate::sequence::{Sequence, SequenceInput};
-use crate::utils::IntOrSlice;
+use crate::utils::{AddInput, IntOrSlice};
 use pyo3::class::basic::CompareOp;
 use pyo3::prelude::*;
 use rayon::prelude::*;
@@ -61,10 +61,16 @@ impl RNASequence {
         self.bool()
     }
 
-    fn __add__(&self, other: MemberOrMembers<RNABase>) -> Self {
-        Self {
-            bases: self.add(other),
-        }
+    fn __add__(&self, other: AddInput<RNABase>) -> PyResult<Self> {
+        Ok(Self {
+            bases: self.add(other, false)?,
+        })
+    }
+
+    fn __radd__(&self, other: AddInput<RNABase>) -> PyResult<Self> {
+        Ok(Self {
+            bases: self.add(other, true)?,
+        })
     }
 
     fn __len__(&self) -> usize {
diff --git a/src/sequence.rs b/src/sequence.rs
index f68e927..f320ed4 100644
--- a/src/sequence.rs
+++ b/src/sequence.rs
@@ -1,5 +1,5 @@
 use crate::member::{MemberOrCode, MemberOrMembers};
-use crate::utils::{IntOrSlice, Wrapper};
+use crate::utils::{AddInput, IntOrSlice, Wrapper};
 use pyo3::class::basic::CompareOp;
 use pyo3::prelude::*;
 use pyo3::pyclass::PyClass;
@@ -72,19 +72,60 @@ where
         }
     }
 
-    fn add(&self, other: MemberOrMembers<T>) -> Vec<T> {
-        match other {
-            MemberOrMembers::Member(member) => {
-                let mut members = self.members().to_vec();
-                members.push(member);
+    fn add(&self, other: AddInput<T>, swap: bool) -> PyResult<Vec<T>>
+    where
+        T: TryFrom<char, Error = PyErr> + Send + Clone,
+    {
+        Ok(match other {
+            AddInput::Member(member) => {
+                let mut members = Vec::with_capacity(self.len() + 1);
+
+                match swap {
+                    true => {
+                        members.push(member);
+                        members.extend(self.members().to_vec());
+                    }
+                    false => {
+                        members.extend(self.members().to_vec());
+                        members.push(member);
+                    }
+                }
+
+                members
+            }
+            AddInput::Members(others) => {
+                let mut members = Vec::with_capacity(self.len() + others.len());
+
+                match swap {
+                    true => {
+                        members.extend(others);
+                        members.extend(self.members().to_vec());
+                    }
+                    false => {
+                        members.extend(self.members().to_vec());
+                        members.extend(others);
+                    }
+                }
+
                 members
             }
-            MemberOrMembers::Sequence(sequence) => {
-                let mut members = self.members().to_vec();
-                members.extend(sequence);
+            AddInput::Codes(codes) => {
+                let mut members = Vec::with_capacity(self.len() + codes.len());
+
+                match swap {
+                    true => {
+                        members.extend(Wrapper::try_from(codes)?.into_inner());
+                        members.extend(self.members().to_vec());
+                    }
+                    false => {
+                        members.extend(self.members().to_vec());
+                        members.extend(Wrapper::try_from(codes)?.into_inner());
+                    }
+                }
+
                 members
             }
-        }
+        })
     }
 
     fn getitem(&self, index_or_slice: IntOrSlice) -> PyResult<MemberOrMembers<T>> {
diff --git a/src/utils.rs b/src/utils.rs
index 69c2e9a..f105f08 100644
--- a/src/utils.rs
+++ b/src/utils.rs
@@ -14,3 +14,10 @@ impl<T> Wrapper<T> {
         self.0
     }
 }
+
+#[derive(FromPyObject)]
+pub enum AddInput<'a, T> {
+    Member(T),
+    Members(Vec<T>),
+    Codes(&'a str),
+}
diff --git a/tests/test_amino_acid.py b/tests/test_amino_acid.py
index baddfa4..41fa811 100644
--- a/tests/test_amino_acid.py
+++ b/tests/test_amino_acid.py
@@ -573,26 +573,41 @@ def test__bool__() -> None:
 @pytest.mark.parametrize(
     "left,right,result",
     [
+        (haem.AminoAcid("M"), haem.AminoAcid("V"), haem.AminoAcidSequence("MV")),
         (
-            haem.AminoAcid.METHIONINE,
-            haem.AminoAcid.VALINE,
-            haem.AminoAcidSequence("MV"),
-        ),
-        (
-            haem.AminoAcid.METHIONINE,
+            haem.AminoAcid("M"),
             haem.AminoAcidSequence("VVR"),
             haem.AminoAcidSequence("MVVR"),
         ),
-        (
-            haem.AminoAcid.METHIONINE,
-            haem.AminoAcidSequence(""),
-            haem.AminoAcidSequence("M"),
-        ),
+        (haem.AminoAcid("M"), haem.AminoAcidSequence(), haem.AminoAcidSequence("M")),
+        (haem.AminoAcid("V"), "M", haem.AminoAcidSequence("VM")),
+        (haem.AminoAcid("V"), "", haem.AminoAcidSequence("V")),
     ],
 )
 def test__add__(
     left: haem.AminoAcid,
-    right: typing.Union[haem.AminoAcid, haem.AminoAcidSequence],
+    right: typing.Union[haem.AminoAcid, haem.AminoAcidSequence, str],
+    result: haem.AminoAcidSequence,
+) -> None:
+    assert left + right == result
+
+
+@pytest.mark.parametrize(
+    "left,right,result",
+    [
+        (
+            haem.AminoAcidSequence("VVR"),
+            haem.AminoAcid("M"),
+            haem.AminoAcidSequence("VVRM"),
+        ),
+        (haem.AminoAcidSequence(), haem.AminoAcid("M"), haem.AminoAcidSequence("M")),
+        ("V", haem.AminoAcid("M"), haem.AminoAcidSequence("VM")),
+        ("", haem.AminoAcid("M"), haem.AminoAcidSequence("M")),
+    ],
+)
+def test__radd__(
+    left: typing.Union[haem.AminoAcidSequence, str],
+    right: haem.AminoAcid,
     result: haem.AminoAcidSequence,
 ) -> None:
     assert left + right == result
diff --git a/tests/test_amino_acid_sequence.py b/tests/test_amino_acid_sequence.py
index d0992d5..3cbcddd 100644
--- a/tests/test_amino_acid_sequence.py
+++ b/tests/test_amino_acid_sequence.py
@@ -136,7 +136,7 @@ def test__bool__(amino_acids: typing.List[haem.AminoAcid], result: bool) -> None
     [
         (
             haem.AminoAcidSequence("MV"),
-            haem.AminoAcid.ARGININE,
+            haem.AminoAcid("R"),
             haem.AminoAcidSequence("MVR"),
         ),
         (
@@ -146,24 +146,43 @@ def test__bool__(amino_acids: typing.List[haem.AminoAcid], result: bool) -> None
         ),
         (
             haem.AminoAcidSequence("MV"),
-            haem.AminoAcidSequence(""),
+            haem.AminoAcidSequence(),
             haem.AminoAcidSequence("MV"),
         ),
-        (
-            haem.AminoAcidSequence(""),
-            haem.AminoAcidSequence(""),
-            haem.AminoAcidSequence(""),
-        ),
-        (
-            haem.AminoAcidSequence(""),
-            haem.AminoAcid.METHIONINE,
-            haem.AminoAcidSequence("M"),
-        ),
+        (haem.AminoAcidSequence(), haem.AminoAcidSequence(), haem.AminoAcidSequence()),
+        (haem.AminoAcidSequence(), haem.AminoAcid("M"), haem.AminoAcidSequence("M")),
+        (haem.AminoAcidSequence(), "", haem.AminoAcidSequence()),
+        (haem.AminoAcidSequence(), "R", haem.AminoAcidSequence("R")),
+        (haem.AminoAcidSequence("MV"), "R", haem.AminoAcidSequence("MVR")),
+        (haem.AminoAcidSequence("MV"), "", haem.AminoAcidSequence("MV")),
     ],
 )
 def test__add__(
     left: haem.AminoAcidSequence,
-    right: typing.Union[haem.AminoAcid, haem.AminoAcidSequence],
+    right: typing.Union[haem.AminoAcid, haem.AminoAcidSequence, str],
+    result: haem.AminoAcidSequence,
+) -> None:
+    assert left + right == result
+
+
+@pytest.mark.parametrize(
+    "left,right,result",
+    [
+        (
+            haem.AminoAcid("R"),
+            haem.AminoAcidSequence("MV"),
+            haem.AminoAcidSequence("RMV"),
+        ),
+        (haem.AminoAcid("M"), haem.AminoAcidSequence(), haem.AminoAcidSequence("M")),
+        ("", haem.AminoAcidSequence(), haem.AminoAcidSequence()),
+        ("R", haem.AminoAcidSequence(), haem.AminoAcidSequence("R")),
+        ("R", haem.AminoAcidSequence("MV"), haem.AminoAcidSequence("RMV")),
+        ("", haem.AminoAcidSequence("MV"), haem.AminoAcidSequence("MV")),
+    ],
+)
+def test__radd__(
+    left: typing.Union[haem.AminoAcid, str],
+    right: haem.AminoAcidSequence,
     result: haem.AminoAcidSequence,
 ) -> None:
     assert left + right == result
@@ -173,11 +192,11 @@ def test__add__(
     "sequence,target,result",
     [
         (haem.AminoAcidSequence("M"), haem.AminoAcid.METHIONINE, True),
-        (haem.AminoAcidSequence(""), haem.AminoAcid.METHIONINE, False),
+        (haem.AminoAcidSequence(), haem.AminoAcid.METHIONINE, False),
         (haem.AminoAcidSequence("V"), haem.AminoAcid.METHIONINE, False),
         (haem.AminoAcidSequence("MVV"), haem.AminoAcid.VALINE, True),
         (haem.AminoAcidSequence("MVV"), haem.AminoAcid.ARGININE, False),
-        (haem.AminoAcidSequence(""), haem.AminoAcidSequence(""), True),
+        (haem.AminoAcidSequence(), haem.AminoAcidSequence(), True),
         (haem.AminoAcidSequence("MVR"), haem.AminoAcidSequence("VR"), True),
         (haem.AminoAcidSequence("MVR"), haem.AminoAcidSequence("RV"), False),
         (haem.AminoAcidSequence("M"), haem.AminoAcidSequence("MM"), False),
@@ -224,7 +243,7 @@ def test__getitem__index_out_of_range(index: int) -> None:
 @pytest.mark.parametrize(
     "sequence,slic,result",
     [
-        (haem.AminoAcidSequence(""), slice(0, 0), haem.AminoAcidSequence("")),
+        (haem.AminoAcidSequence(), slice(0, 0), haem.AminoAcidSequence()),
         (haem.AminoAcidSequence("MVR"), slice(0, 2), haem.AminoAcidSequence("MV")),
         (haem.AminoAcidSequence("MVR"), slice(1, 3), haem.AminoAcidSequence("VR")),
         (haem.AminoAcidSequence("MVR"), slice(0, 3), haem.AminoAcidSequence("MVR")),
@@ -280,7 +299,7 @@ def test__iter__(amino_acids: typing.List[haem.AminoAcid]) -> None:
 @pytest.mark.parametrize(
     "sequence,target,total",
     [
-        (haem.AminoAcidSequence(""), haem.AminoAcid.VALINE, 0),
+        (haem.AminoAcidSequence(), haem.AminoAcid.VALINE, 0),
         (haem.AminoAcidSequence("MVRV"), haem.AminoAcid.VALINE, 2),
         (haem.AminoAcidSequence("MVRV"), "V", 2),
     ],
diff --git a/tests/test_dna_base.py b/tests/test_dna_base.py
index db60e7a..9d05a3f 100644
--- a/tests/test_dna_base.py
+++ b/tests/test_dna_base.py
@@ -174,14 +174,16 @@ def test__bool__(base: haem.DNABase, result: bool) -> None:
 @pytest.mark.parametrize(
     "left,right,result",
     [
-        (haem.DNABase.ADENINE, haem.DNABase.GAP, haem.DNASequence("A-")),
-        (haem.DNABase.ADENINE, haem.DNASequence("CTT"), haem.DNASequence("ACTT")),
-        (haem.DNABase.ADENINE, haem.DNASequence(""), haem.DNASequence("A")),
+        (haem.DNABase("A"), haem.DNABase("-"), haem.DNASequence("A-")),
+        (haem.DNABase("A"), haem.DNASequence("CTT"), haem.DNASequence("ACTT")),
+        (haem.DNABase("A"), haem.DNASequence(), haem.DNASequence("A")),
+        (haem.DNABase("A"), "", haem.DNASequence("A")),
+        (haem.DNABase("A"), haem.DNASequence("T"), haem.DNASequence("AT")),
     ],
 )
 def test__add__(
     left: haem.DNABase,
-    right: typing.Union[haem.DNABase, haem.DNASequence],
+    right: typing.Union[haem.DNABase, haem.DNASequence, str],
     result: haem.DNASequence,
 ) -> None:
     assert left + right == result
@@ -190,12 +192,14 @@ def test__add__(
 @pytest.mark.parametrize(
     "left,right,result",
     [
-        (haem.DNASequence("CTT"), haem.DNABase.ADENINE, haem.DNASequence("CTTA")),
-        (haem.DNASequence(""), haem.DNABase.ADENINE, haem.DNASequence("A")),
+        (haem.DNASequence("CTT"), haem.DNABase("A"), haem.DNASequence("CTTA")),
+        (haem.DNASequence(), haem.DNABase("A"), haem.DNASequence("A")),
+        ("", haem.DNABase("A"), haem.DNASequence("A")),
+        ("T", haem.DNABase("A"), haem.DNASequence("TA")),
     ],
 )
 def test__radd__(
-    left: typing.Union[haem.DNABase, haem.DNASequence],
+    left: typing.Union[haem.DNASequence, str],
     right: haem.DNABase,
     result: haem.DNASequence,
 ) -> None:
diff --git a/tests/test_dna_sequence.py b/tests/test_dna_sequence.py
index 95d98b2..9f98a84 100644
--- a/tests/test_dna_sequence.py
+++ b/tests/test_dna_sequence.py
@@ -66,7 +66,7 @@ def test__new__sequence_str() -> None:
 @pytest.mark.parametrize(
     "sequence,complement",
     [
-        (haem.DNASequence(""), haem.DNASequence("")),
+        (haem.DNASequence(), haem.DNASequence()),
         (haem.DNASequence("A"), haem.DNASequence("T")),
         (haem.DNASequence("ACGT"), haem.DNASequence("TGCA")),
     ],
@@ -79,7 +79,7 @@ def test_complement(sequence: haem.DNASequence, complement: haem.DNASequence) ->
 @pytest.mark.parametrize(
     "dna_sequence,rna_sequence",
     [
-        (haem.DNASequence(""), haem.RNASequence("")),
+        (haem.DNASequence(), haem.RNASequence()),
         (haem.DNASequence("ACGT"), haem.RNASequence("ACGU")),
     ],
 )
@@ -148,14 +148,18 @@ def test__bool__(bases: typing.List[haem.DNABase], result: bool) -> None:
     [
         (haem.DNASequence("A-"), haem.DNABase.GUANINE, haem.DNASequence("A-G")),
         (haem.DNASequence("A-"), haem.DNASequence("CTT"), haem.DNASequence("A-CTT")),
-        (haem.DNASequence("A-"), haem.DNASequence(""), haem.DNASequence("A-")),
-        (haem.DNASequence(""), haem.DNASequence(""), haem.DNASequence("")),
-        (haem.DNASequence(""), haem.DNABase.GUANINE, haem.DNASequence("G")),
+        (haem.DNASequence("A-"), haem.DNASequence(), haem.DNASequence("A-")),
+        (haem.DNASequence(), haem.DNASequence(), haem.DNASequence()),
+        (haem.DNASequence(), haem.DNABase.GUANINE, haem.DNASequence("G")),
+        (haem.DNASequence("A-"), "", haem.DNASequence("A-")),
+        (haem.DNASequence("A-"), "GT", haem.DNASequence("A-GT")),
+        (haem.DNASequence(), "", haem.DNASequence()),
+        (haem.DNASequence(), "TG", haem.DNASequence("TG")),
     ],
 )
 def test__add__(
     left: haem.DNASequence,
-    right: typing.Union[haem.DNABase, haem.DNASequence],
+    right: typing.Union[haem.DNABase, haem.DNASequence, str],
     result: haem.DNASequence,
 ) -> None:
     assert left + right == result
@@ -164,13 +168,17 @@ def test__add__(
 @pytest.mark.parametrize(
     "left,right,result",
     [
-        (haem.DNABase.GUANINE, haem.DNASequence("A-"), haem.DNASequence("GA-")),
-        (haem.DNABase.GUANINE, haem.DNASequence(""), haem.DNASequence("G")),
+        (haem.DNABase("G"), haem.DNASequence("A-"), haem.DNASequence("GA-")),
+        (haem.DNABase("G"), haem.DNASequence(), haem.DNASequence("G")),
+        ("", haem.DNASequence("A-"), haem.DNASequence("A-")),
+        ("GT", haem.DNASequence("A-"), haem.DNASequence("GTA-")),
+        ("", haem.DNASequence(), haem.DNASequence()),
+        ("TG", haem.DNASequence(), haem.DNASequence("TG")),
     ],
 )
 def test__radd__(
-    left: haem.DNASequence,
-    right: typing.Union[haem.DNABase, haem.DNASequence],
+    left: typing.Union[haem.DNABase, str],
+    right: haem.DNASequence,
     result: haem.DNASequence,
 ) -> None:
     assert left + right == result
@@ -180,11 +188,11 @@ def test__radd__(
     "sequence,target,result",
     [
         (haem.DNASequence("A"), haem.DNABase.ADENINE, True),
-        (haem.DNASequence(""), haem.DNABase.ADENINE, False),
+        (haem.DNASequence(), haem.DNABase.ADENINE, False),
         (haem.DNASequence("G"), haem.DNABase.ADENINE, False),
         (haem.DNASequence("AGCG"), haem.DNABase.GUANINE, True),
         (haem.DNASequence("AGCG"), haem.DNABase.THYMINE, False),
-        (haem.DNASequence(""), haem.DNASequence(""), True),
+        (haem.DNASequence(), haem.DNASequence(), True),
         (haem.DNASequence("AGT"), haem.DNASequence("GT"), True),
         (haem.DNASequence("AGT"), haem.DNASequence("TG"), False),
         (haem.DNASequence("A"), haem.DNASequence("AA"), False),
@@ -231,7 +239,7 @@ def test__getitem__index_out_of_range(index: int) -> None:
 @pytest.mark.parametrize(
     "sequence,slic,result",
     [
-        (haem.DNASequence(""), slice(0, 0), haem.DNASequence("")),
+        (haem.DNASequence(), slice(0, 0), haem.DNASequence()),
         (haem.DNASequence("GAT"), slice(0, 2), haem.DNASequence("GA")),
         (haem.DNASequence("GAT"), slice(1, 3), haem.DNASequence("AT")),
         (haem.DNASequence("GAT"), slice(0, 3), haem.DNASequence("GAT")),
@@ -270,7 +278,7 @@ def test__iter__(bases: typing.List[haem.DNABase]) -> None:
 @pytest.mark.parametrize(
     "sequence,target,total",
     [
-        (haem.DNASequence(""), haem.DNABase.ADENINE, 0),
+        (haem.DNASequence(), haem.DNABase.ADENINE, 0),
         (haem.DNASequence("AGCG"), haem.DNABase.GUANINE, 2),
         (haem.DNASequence("AGCG"), "G", 2),
     ],
diff --git a/tests/test_rna_base.py b/tests/test_rna_base.py
index 1a3cb54..6f5130f 100644
--- a/tests/test_rna_base.py
+++ b/tests/test_rna_base.py
@@ -174,14 +174,16 @@ def test__bool__(base: haem.RNABase, result: bool) -> None:
 @pytest.mark.parametrize(
     "left,right,result",
     [
-        (haem.RNABase.ADENINE, haem.RNABase.GAP, haem.RNASequence("A-")),
+        (haem.RNABase.ADENINE, haem.RNABase("-"), haem.RNASequence("A-")),
         (haem.RNABase.ADENINE, haem.RNASequence("CUU"), haem.RNASequence("ACUU")),
-        (haem.RNABase.ADENINE, haem.RNASequence(""), haem.RNASequence("A")),
+        (haem.RNABase.ADENINE, haem.RNASequence(), haem.RNASequence("A")),
+        (haem.RNABase.ADENINE, "-", haem.RNASequence("A-")),
+        (haem.RNABase.ADENINE, "", haem.RNASequence("A")),
     ],
 )
 def test__add__(
     left: haem.RNABase,
-    right: typing.Union[haem.RNABase, haem.RNASequence],
+    right: typing.Union[haem.RNABase, haem.RNASequence, str],
     result: haem.RNASequence,
 ) -> None:
     assert left + right == result
@@ -190,12 +192,14 @@ def test__add__(
 @pytest.mark.parametrize(
     "left,right,result",
     [
-        (haem.RNASequence("CUU"), haem.RNABase.ADENINE, haem.RNASequence("CUUA")),
-        (haem.RNASequence(""), haem.RNABase.ADENINE, haem.RNASequence("A")),
+        (haem.RNASequence("CUU"), haem.RNABase("A"), haem.RNASequence("CUUA")),
+        (haem.RNASequence(), haem.RNABase("A"), haem.RNASequence("A")),
+        ("C", haem.RNABase("A"), haem.RNASequence("CA")),
+        ("", haem.RNABase("A"), haem.RNASequence("A")),
     ],
 )
 def test__radd__(
-    left: typing.Union[haem.RNABase, haem.RNASequence],
+    left: typing.Union[haem.RNASequence, str],
     right: haem.RNABase,
     result: haem.RNASequence,
 ) -> None:
diff --git a/tests/test_rna_sequence.py b/tests/test_rna_sequence.py
index cf91c17..dfaad7c 100644
--- a/tests/test_rna_sequence.py
+++ b/tests/test_rna_sequence.py
@@ -66,7 +66,7 @@ def test__new__sequence_str() -> None:
 @pytest.mark.parametrize(
     "sequence,complement",
     [
-        (haem.RNASequence(""), haem.RNASequence("")),
+        (haem.RNASequence(), haem.RNASequence()),
         (haem.RNASequence("A"), haem.RNASequence("U")),
         (haem.RNASequence("ACGU"), haem.RNASequence("UGCA")),
     ],
@@ -79,7 +79,7 @@ def test_complement(sequence: haem.RNASequence, complement: haem.RNASequence) ->
 @pytest.mark.parametrize(
     "rna_sequence,dna_sequence",
     [
-        (haem.RNASequence(""), haem.DNASequence("")),
+        (haem.RNASequence(), haem.DNASequence("")),
         (haem.RNASequence("ACGU"), haem.DNASequence("ACGT")),
     ],
 )
@@ -148,14 +148,18 @@ def test__bool__(bases: typing.List[haem.RNABase], result: bool) -> None:
     [
         (haem.RNASequence("A-"), haem.RNABase.GUANINE, haem.RNASequence("A-G")),
         (haem.RNASequence("A-"), haem.RNASequence("CUU"), haem.RNASequence("A-CUU")),
-        (haem.RNASequence("A-"), haem.RNASequence(""), haem.RNASequence("A-")),
-        (haem.RNASequence(""), haem.RNASequence(""), haem.RNASequence("")),
-        (haem.RNASequence(""), haem.RNABase.GUANINE, haem.RNASequence("G")),
+        (haem.RNASequence("A-"), haem.RNASequence(), haem.RNASequence("A-")),
+        (haem.RNASequence(), haem.RNASequence(), haem.RNASequence()),
+        (haem.RNASequence(), haem.RNABase.GUANINE, haem.RNASequence("G")),
+        (haem.RNASequence("A-"), "", haem.RNASequence("A-")),
+        (haem.RNASequence("A-"), "GU", haem.RNASequence("A-GU")),
+        (haem.RNASequence(), "", haem.RNASequence()),
+        (haem.RNASequence(), "UG", haem.RNASequence("UG")),
     ],
 )
 def test__add__(
     left: haem.RNASequence,
-    right: typing.Union[haem.RNABase, haem.RNASequence],
+    right: typing.Union[haem.RNABase, haem.RNASequence, str],
     result: haem.RNASequence,
 ) -> None:
     assert left + right == result
@@ -165,12 +169,16 @@ def test__add__(
     "left,right,result",
     [
         (haem.RNABase.GUANINE, haem.RNASequence("A-"), haem.RNASequence("GA-")),
-        (haem.RNABase.GUANINE, haem.RNASequence(""), haem.RNASequence("G")),
+        (haem.RNABase.GUANINE, haem.RNASequence(), haem.RNASequence("G")),
+        ("", haem.RNASequence("A-"), haem.RNASequence("A-")),
+        ("GU", haem.RNASequence("A-"), haem.RNASequence("GUA-")),
+        ("", haem.RNASequence(), haem.RNASequence()),
+        ("UG", haem.RNASequence(), haem.RNASequence("UG")),
     ],
 )
 def test__radd__(
-    left: haem.RNASequence,
-    right: typing.Union[haem.RNABase, haem.RNASequence],
+    left: typing.Union[haem.RNABase, str],
+    right: haem.RNASequence,
     result: haem.RNASequence,
 ) -> None:
     assert left + right == result
@@ -180,11 +188,11 @@ def test__radd__(
     "sequence,target,result",
     [
         (haem.RNASequence("A"), haem.RNABase.ADENINE, True),
-        (haem.RNASequence(""), haem.RNABase.ADENINE, False),
+        (haem.RNASequence(), haem.RNABase.ADENINE, False),
         (haem.RNASequence("G"), haem.RNABase.ADENINE, False),
         (haem.RNASequence("AGCG"), haem.RNABase.GUANINE, True),
         (haem.RNASequence("AGCG"), haem.RNABase.URACIL, False),
-        (haem.RNASequence(""), haem.RNASequence(""), True),
+        (haem.RNASequence(), haem.RNASequence(), True),
         (haem.RNASequence("AGU"), haem.RNASequence("GU"), True),
         (haem.RNASequence("AGU"), haem.RNASequence("UG"), False),
         (haem.RNASequence("A"), haem.RNASequence("AA"), False),
@@ -231,7 +239,7 @@ def test__getitem__index_out_of_range(index: int) -> None:
 @pytest.mark.parametrize(
     "sequence,slic,result",
     [
-        (haem.RNASequence(""), slice(0, 0), haem.RNASequence("")),
+        (haem.RNASequence(), slice(0, 0), haem.RNASequence()),
         (haem.RNASequence("GAU"), slice(0, 2), haem.RNASequence("GA")),
         (haem.RNASequence("GAU"), slice(1, 3), haem.RNASequence("AU")),
         (haem.RNASequence("GAU"), slice(0, 3), haem.RNASequence("GAU")),
@@ -270,7 +278,7 @@ def test__iter__(bases: typing.List[haem.RNABase]) -> None:
 @pytest.mark.parametrize(
     "sequence,target,total",
     [
-        (haem.RNASequence(""), haem.RNABase.ADENINE, 0),
+        (haem.RNASequence(), haem.RNABase.ADENINE, 0),
         (haem.RNASequence("AGCG"), haem.RNABase.GUANINE, 2),
         (haem.RNASequence("AGCG"), "G", 2),
     ],