diff --git a/src/cryptography/x509.py b/src/cryptography/x509.py
index a786c70..1635a51 100644
--- a/src/cryptography/x509.py
+++ b/src/cryptography/x509.py
@@ -1169,6 +1169,9 @@
 
     @abc.abstractmethod
     def public_bytes(self, encoding):
+        """
+        Serializes the certificate to PEM or DER format.
+        """
         pass