def METHOD_NAME():
    extension_definition = stix2.v21.ExtensionDefinition(
        id=EXTENSION_DEFINITION_IDS[0],
        created_by_ref="identity--11b76a96-5d2b-45e0-8a5a-f6994f370731",
        created="2014-02-20T09:16:08.000Z",
        modified="2014-02-20T09:16:08.000Z",
        name="New SDO 1",
        description="This schema creates a new object type called my-favorite-sdo-1",
        schema="https://www.example.com/schema-my-favorite-sdo-1/v1/",
        version="1.2.1",
        extension_types=["new-sdo"],
    )
    assert extension_definition.serialize(pretty=True) == EXPECTED