diff --git a/src/Message.php b/src/Message.php
index e39f13a..cc385ed 100644
--- a/src/Message.php
+++ b/src/Message.php
@@ -4,6 +4,7 @@
 use paragraph1\phpFCM\Recipient\Recipient;
 use paragraph1\phpFCM\Recipient\Topic;
 use paragraph1\phpFCM\Recipient\Device;
+use paragraph1\phpFCM\Recipient\GroupTopic;
 
 /**
  * @author palbertini
@@ -28,6 +29,7 @@ class Message implements \JsonSerializable
     private $recipientType;
     private $timeToLive;
     private $delayWhileIdle;
+    private $mutableContent;
 
     /**
      * Represents the app's "Send-to-Sync" message.
@@ -47,7 +49,7 @@ class Message implements \JsonSerializable
      */
     public function addRecipient(Recipient $recipient)
     {
-        if (!$recipient instanceof Device && !$recipient instanceof Topic) {
+        if (!$recipient instanceof Device && !$recipient instanceof Topic && !$recipient instanceof GroupTopic) {
             throw new \UnexpectedValueException('currently phpFCM only supports topic and single device messages');
         }
 
@@ -123,6 +125,10 @@ public function setDelayWhileIdle($delayWhileIdle)
         return $this;
     }
 
+    public function setMutableContent()
+    {
+        $this->mutableContent = 1;
+    }
     /**
      * @see https://firebase.google.com/docs/cloud-messaging/concept-options#collapsible_and_non-collapsible_messages
      *
@@ -166,6 +172,11 @@ public function jsonSerialize()
         if ($this->delayWhileIdle) {
             $jsonData['delay_while_idle'] = (bool)$this->delayWhileIdle;
         }
+
+        if ($this->mutableContent) {
+            $jsonData['mutable_content'] = (bool)$this->mutableContent;
+        }
+
         if ($this->contentAvailableFlag === TRUE) {
             $jsonData['content_available'] = TRUE;
         }
@@ -187,6 +198,18 @@ function (Topic $topic) { return sprintf("'%s' in topics", $topic->getIdentifier
                 }
                 $jsonData['to'] = sprintf('/topics/%s', current($this->recipients)->getIdentifier());
                 break;
+                
+            case GroupTopic::class:
+                if (count($this->recipients) > 1) {
+                    $topics = array_map(
+                        function (GroupTopic $topic) { return sprintf("'%s' in topics", $topic->getIdentifier()); },
+                        $this->recipients
+                    );
+                    $jsonData['condition'] = implode(' && ', $topics);
+                    break;
+                }
+                $jsonData['to'] = sprintf('/topics/%s', current($this->recipients)->getIdentifier());
+                break;
             default:
                 if (count($this->recipients) === 1) {
                     $jsonData['to'] = current($this->recipients)->getIdentifier();
@@ -199,4 +222,4 @@ function (Topic $topic) { return sprintf("'%s' in topics", $topic->getIdentifier
                 }
         }
     }
-}
\ No newline at end of file
+}
diff --git a/src/Recipient/GroupTopic.php b/src/Recipient/GroupTopic.php
new file mode 100644
index 0000000..c88934e
--- /dev/null
+++ b/src/Recipient/GroupTopic.php
@@ -0,0 +1,17 @@
+<?php
+namespace paragraph1\phpFCM\Recipient;
+
+class GroupTopic implements Recipient
+{
+    private $name;
+
+    public function __construct($name)
+    {
+        $this->name = $name;
+    }
+
+    public function getIdentifier()
+    {
+        return $this->name;
+    }
+}
\ No newline at end of file
diff --git a/tests/MessageTest.php b/tests/MessageTest.php
index c52fa96..e955e85 100644
--- a/tests/MessageTest.php
+++ b/tests/MessageTest.php
@@ -133,6 +133,18 @@ public function testJsonEncodeHandlesDelayIdle()
         );
     }
 
+    public function testJsonEncodeHandlesMutableContent()
+    {
+        $body = '{"to":"\/topics\/testing","data":{"foo":"bar"},"priority":"high","mutable_content":true}';
+        $this->fixture->setData(['foo' => 'bar'])
+            ->setMutableContent();
+        $this->fixture->addRecipient(new Topic('testing'));
+        $this->assertSame(
+            $body,
+            json_encode($this->fixture)
+        );
+    }
+
     public function testAddingNewAndUnknownRecipientTypesYieldsException()
     {
         $this->setExpectedException(\UnexpectedValueException::class);