diff --git a/stix2/v21/observables.py b/stix2/v21/observables.py
index e39c709..a68e3d5 100644
--- a/stix2/v21/observables.py
+++ b/stix2/v21/observables.py
@@ -952,8 +952,10 @@ def CustomObservable(type='x-custom-observable', properties=None, id_contrib_pro
                 [('type', TypeProperty(type, spec_version='2.1'))],
                 [('spec_version', StringProperty(fixed='2.1'))],
                 [('id', IDProperty(type, spec_version='2.1'))],
-                [('spec_version', StringProperty(fixed='2.1'))],
                 properties,
+                [('object_marking_refs', ListProperty(ReferenceProperty(valid_types='marking-definition', spec_version='2.1')))],
+                [('granular_markings', ListProperty(GranularMarking))],
+                [('defanged', BooleanProperty(default=lambda: False))],
                 [('extensions', ExtensionsProperty(spec_version='2.1'))],
             ]),
         )