diff --git a/stix2/base.py b/stix2/base.py
index e018059..5af3607 100644
--- a/stix2/base.py
+++ b/stix2/base.py
@@ -79,11 +79,6 @@ def get_required_properties(properties):
 class _STIXBase(Mapping):
     """Base class for STIX object types"""
 
-    def get_class_version(self):
-        module_name = self.__class__.__module__
-        module_parts = module_name.split(".")
-        return module_parts[1]
-
     def object_properties(self):
         props = set(self._properties.keys())
         custom_props = list(set(self._inner.keys()) - props)
@@ -175,7 +170,7 @@ class _STIXBase(Mapping):
 
         if custom_props or extra_kwargs:
             self._allow_custom = True
-            if self.get_class_version() == "v21":
+            if self._spec_version == "2.1":
                 all_custom_prop_names = extra_kwargs
                 all_custom_prop_names.extend(list(custom_props.keys()))
                 for prop_name in all_custom_prop_names:
diff --git a/stix2/custom.py b/stix2/custom.py
index bd2fe1d..687618d 100644
--- a/stix2/custom.py
+++ b/stix2/custom.py
@@ -3,18 +3,18 @@ import re
 
 import six
 
-from .base import _cls_init, _Extension, _Observable, _STIXBase
+from .base import _cls_init
 from .core import (
-    STIXDomainObject, _register_marking, _register_object,
-    _register_observable, _register_observable_extension,
+    _register_marking, _register_object, _register_observable,
+    _register_observable_extension,
 )
 from .utils import (
     PREFIX_21_REGEX, TYPE_21_REGEX, TYPE_REGEX, get_class_hierarchy_names,
 )
 
 
-def _custom_object_builder(cls, type, properties, version):
-    class _CustomObject(cls, STIXDomainObject):
+def _custom_object_builder(cls, type, properties, version, base_class):
+    class _CustomObject(cls, base_class):
 
         if version == "2.0":
             if not re.match(TYPE_REGEX, type):
@@ -48,16 +48,16 @@ def _custom_object_builder(cls, type, properties, version):
         _properties = OrderedDict(properties)
 
         def __init__(self, **kwargs):
-            _STIXBase.__init__(self, **kwargs)
+            base_class.__init__(self, **kwargs)
             _cls_init(cls, self, kwargs)
 
     _register_object(_CustomObject, version=version)
     return _CustomObject
 
 
-def _custom_marking_builder(cls, type, properties, version):
+def _custom_marking_builder(cls, type, properties, version, base_class):
 
-    class _CustomMarking(cls, _STIXBase):
+    class _CustomMarking(cls, base_class):
 
         if not properties or not isinstance(properties, list):
             raise ValueError("Must supply a list, containing tuples. For example, [('property1', IntegerProperty())]")
@@ -66,18 +66,18 @@ def _custom_marking_builder(cls, type, properties, version):
         _properties = OrderedDict(properties)
 
         def __init__(self, **kwargs):
-            _STIXBase.__init__(self, **kwargs)
+            base_class.__init__(self, **kwargs)
             _cls_init(cls, self, kwargs)
 
     _register_marking(_CustomMarking, version=version)
     return _CustomMarking
 
 
-def _custom_observable_builder(cls, type, properties, version, id_contrib_props=None):
+def _custom_observable_builder(cls, type, properties, version, base_class, id_contrib_props=None):
     if id_contrib_props is None:
         id_contrib_props = []
 
-    class _CustomObservable(cls, _Observable):
+    class _CustomObservable(cls, base_class):
 
         if version == "2.0":
             if not re.match(TYPE_REGEX, type):
@@ -137,14 +137,14 @@ def _custom_observable_builder(cls, type, properties, version, id_contrib_props=
             _id_contributing_properties = id_contrib_props
 
         def __init__(self, **kwargs):
-            _Observable.__init__(self, **kwargs)
+            base_class.__init__(self, **kwargs)
             _cls_init(cls, self, kwargs)
 
     _register_observable(_CustomObservable, version=version)
     return _CustomObservable
 
 
-def _custom_extension_builder(cls, observable, type, properties, version):
+def _custom_extension_builder(cls, observable, type, properties, version, base_class):
 
     try:
         prop_dict = OrderedDict(properties)
@@ -158,13 +158,13 @@ def _custom_extension_builder(cls, observable, type, properties, version):
             e,
         )
 
-    class _CustomExtension(cls, _Extension):
+    class _CustomExtension(cls, base_class):
 
         _type = type
         _properties = prop_dict
 
         def __init__(self, **kwargs):
-            _Extension.__init__(self, **kwargs)
+            base_class.__init__(self, **kwargs)
             _cls_init(cls, self, kwargs)
 
     _register_observable_extension(observable, _CustomExtension, version=version)
diff --git a/stix2/v20/base.py b/stix2/v20/base.py
new file mode 100644
index 0000000..e32069a
--- /dev/null
+++ b/stix2/v20/base.py
@@ -0,0 +1,24 @@
+"""Base classes for STIX 2.0 type definitions."""
+
+from ..base import _Extension, _Observable, _STIXBase
+from ..core import STIXDomainObject, STIXRelationshipObject
+
+
+class _STIXBase20(_STIXBase):
+    _spec_version = "2.0"
+
+
+class _Observable(_Observable, _STIXBase20):
+    pass
+
+
+class _Extension(_Extension, _STIXBase20):
+    pass
+
+
+class STIXDomainObject(STIXDomainObject, _STIXBase20):
+    pass
+
+
+class STIXRelationshipObject(STIXRelationshipObject, _STIXBase20):
+    pass
diff --git a/stix2/v20/bundle.py b/stix2/v20/bundle.py
index ffd70ea..b811a79 100644
--- a/stix2/v20/bundle.py
+++ b/stix2/v20/bundle.py
@@ -2,13 +2,13 @@
 
 from collections import OrderedDict
 
-from ..base import _STIXBase
 from ..properties import (
     IDProperty, ListProperty, STIXObjectProperty, StringProperty, TypeProperty,
 )
+from .base import _STIXBase20
 
 
-class Bundle(_STIXBase):
+class Bundle(_STIXBase20):
     """For more detailed information on this object's properties, see
     `the STIX 2.0 specification <http://docs.oasis-open.org/cti/stix/v2.0/cs01/part1-stix-core/stix-v2.0-cs01-part1-stix-core.html#_Toc496709293>`__.
     """
diff --git a/stix2/v20/common.py b/stix2/v20/common.py
index a1ffa60..55f2c3e 100644
--- a/stix2/v20/common.py
+++ b/stix2/v20/common.py
@@ -5,7 +5,6 @@ import copy
 
 import six
 
-from ..base import _STIXBase
 from ..custom import _custom_marking_builder
 from ..markings import _MarkingsMixin
 from ..markings.utils import check_tlp_marking
@@ -14,6 +13,7 @@ from ..properties import (
     SelectorProperty, StringProperty, TimestampProperty, TypeProperty,
 )
 from ..utils import NOW, _get_dict
+from .base import _STIXBase20
 
 
 def _should_set_millisecond(cr, marking_type):
@@ -31,7 +31,7 @@ def _should_set_millisecond(cr, marking_type):
     return False
 
 
-class ExternalReference(_STIXBase):
+class ExternalReference(_STIXBase20):
     """For more detailed information on this object's properties, see
     `the STIX 2.0 specification <http://docs.oasis-open.org/cti/stix/v2.0/cs01/part1-stix-core/stix-v2.0-cs01-part1-stix-core.html#_Toc496709261>`__.
     """
@@ -49,7 +49,7 @@ class ExternalReference(_STIXBase):
         self._check_at_least_one_property(['description', 'external_id', 'url'])
 
 
-class KillChainPhase(_STIXBase):
+class KillChainPhase(_STIXBase20):
     """For more detailed information on this object's properties, see
     `the STIX 2.0 specification <http://docs.oasis-open.org/cti/stix/v2.0/cs01/part1-stix-core/stix-v2.0-cs01-part1-stix-core.html#_Toc496709267>`__.
     """
@@ -60,7 +60,7 @@ class KillChainPhase(_STIXBase):
     ])
 
 
-class GranularMarking(_STIXBase):
+class GranularMarking(_STIXBase20):
     """For more detailed information on this object's properties, see
     `the STIX 2.0 specification <http://docs.oasis-open.org/cti/stix/v2.0/cs01/part1-stix-core/stix-v2.0-cs01-part1-stix-core.html#_Toc496709290>`__.
     """
@@ -71,7 +71,7 @@ class GranularMarking(_STIXBase):
     ])
 
 
-class TLPMarking(_STIXBase):
+class TLPMarking(_STIXBase20):
     """For more detailed information on this object's properties, see
     `the STIX 2.0 specification <http://docs.oasis-open.org/cti/stix/v2.0/cs01/part1-stix-core/stix-v2.0-cs01-part1-stix-core.html#_Toc496709287>`__.
     """
@@ -83,7 +83,7 @@ class TLPMarking(_STIXBase):
     ])
 
 
-class StatementMarking(_STIXBase):
+class StatementMarking(_STIXBase20):
     """For more detailed information on this object's properties, see
     `the STIX 2.0 specification <http://docs.oasis-open.org/cti/stix/v2.0/cs01/part1-stix-core/stix-v2.0-cs01-part1-stix-core.html#_Toc496709286>`__.
     """
@@ -113,7 +113,7 @@ class MarkingProperty(Property):
             raise ValueError("must be a Statement, TLP Marking or a registered marking.")
 
 
-class MarkingDefinition(_STIXBase, _MarkingsMixin):
+class MarkingDefinition(_STIXBase20, _MarkingsMixin):
     """For more detailed information on this object's properties, see
     `the STIX 2.0 specification <http://docs.oasis-open.org/cti/stix/v2.0/cs01/part1-stix-core/stix-v2.0-cs01-part1-stix-core.html#_Toc496709284>`__.
     """
@@ -182,7 +182,7 @@ def CustomMarking(type='x-custom-marking', properties=None):
 
     """
     def wrapper(cls):
-        return _custom_marking_builder(cls, type, properties, '2.0')
+        return _custom_marking_builder(cls, type, properties, '2.0', _STIXBase20)
     return wrapper
 
 
diff --git a/stix2/v20/observables.py b/stix2/v20/observables.py
index 81e2c48..eb9bfde 100644
--- a/stix2/v20/observables.py
+++ b/stix2/v20/observables.py
@@ -1,14 +1,13 @@
 """STIX 2.0 Cyber Observable Objects.
 
 Embedded observable object types, such as Email MIME Component, which is
-embedded in Email Message objects, inherit from ``_STIXBase`` instead of
-Observable and do not have a ``_type`` attribute.
+embedded in Email Message objects, inherit from ``_STIXBase20`` instead of
+_Observable and do not have a ``_type`` attribute.
 """
 
 from collections import OrderedDict
 import itertools
 
-from ..base import _Extension, _Observable, _STIXBase
 from ..custom import _custom_extension_builder, _custom_observable_builder
 from ..exceptions import AtLeastOnePropertyError, DependentPropertiesError
 from ..properties import (
@@ -17,6 +16,7 @@ from ..properties import (
     HashesProperty, HexProperty, IntegerProperty, ListProperty,
     ObjectReferenceProperty, StringProperty, TimestampProperty, TypeProperty,
 )
+from .base import _Extension, _Observable, _STIXBase20
 
 
 class Artifact(_Observable):
@@ -103,7 +103,7 @@ class EmailAddress(_Observable):
     ])
 
 
-class EmailMIMEComponent(_STIXBase):
+class EmailMIMEComponent(_STIXBase20):
     """For more detailed information on this object's properties, see
     `the STIX 2.0 specification <http://docs.oasis-open.org/cti/stix/v2.0/cs01/part4-cyber-observable-objects/stix-v2.0-cs01-part4-cyber-observable-objects.html#_Toc496716231>`__.
     """  # noqa
@@ -166,7 +166,7 @@ class ArchiveExt(_Extension):
     ])
 
 
-class AlternateDataStream(_STIXBase):
+class AlternateDataStream(_STIXBase20):
     """For more detailed information on this object's properties, see
     `the STIX 2.0 specification <http://docs.oasis-open.org/cti/stix/v2.0/cs01/part4-cyber-observable-objects/stix-v2.0-cs01-part4-cyber-observable-objects.html#_Toc496716239>`__.
     """  # noqa
@@ -220,7 +220,7 @@ class RasterImageExt(_Extension):
     ])
 
 
-class WindowsPEOptionalHeaderType(_STIXBase):
+class WindowsPEOptionalHeaderType(_STIXBase20):
     """For more detailed information on this object's properties, see
     `the STIX 2.0 specification <http://docs.oasis-open.org/cti/stix/v2.0/cs01/part4-cyber-observable-objects/stix-v2.0-cs01-part4-cyber-observable-objects.html#_Toc496716248>`__.
     """  # noqa
@@ -264,7 +264,7 @@ class WindowsPEOptionalHeaderType(_STIXBase):
         self._check_at_least_one_property()
 
 
-class WindowsPESection(_STIXBase):
+class WindowsPESection(_STIXBase20):
     """For more detailed information on this object's properties, see
     `the STIX 2.0 specification <http://docs.oasis-open.org/cti/stix/v2.0/cs01/part4-cyber-observable-objects/stix-v2.0-cs01-part4-cyber-observable-objects.html#_Toc496716250>`__.
     """  # noqa
@@ -677,7 +677,7 @@ class UserAccount(_Observable):
     ])
 
 
-class WindowsRegistryValueType(_STIXBase):
+class WindowsRegistryValueType(_STIXBase20):
     """For more detailed information on this object's properties, see
     `the STIX 2.0 specification <http://docs.oasis-open.org/cti/stix/v2.0/cs01/part4-cyber-observable-objects/stix-v2.0-cs01-part4-cyber-observable-objects.html#_Toc496716293>`__.
     """  # noqa
@@ -724,7 +724,7 @@ class WindowsRegistryKey(_Observable):
     ])
 
 
-class X509V3ExtenstionsType(_STIXBase):
+class X509V3ExtenstionsType(_STIXBase20):
     """For more detailed information on this object's properties, see
     `the STIX 2.0 specification <http://docs.oasis-open.org/cti/stix/v2.0/cs01/part4-cyber-observable-objects/stix-v2.0-cs01-part4-cyber-observable-objects.html#_Toc496716298>`__.
     """  # noqa
@@ -795,7 +795,7 @@ def CustomObservable(type='x-custom-observable', properties=None):
             properties,
             [('extensions', ExtensionsProperty(spec_version="2.0", enclosing_type=type))],
         ]))
-        return _custom_observable_builder(cls, type, _properties, '2.0')
+        return _custom_observable_builder(cls, type, _properties, '2.0', _Observable)
     return wrapper
 
 
@@ -803,5 +803,5 @@ def CustomExtension(observable=None, type='x-custom-observable-ext', properties=
     """Decorator for custom extensions to STIX Cyber Observables.
     """
     def wrapper(cls):
-        return _custom_extension_builder(cls, observable, type, properties, '2.0')
+        return _custom_extension_builder(cls, observable, type, properties, '2.0', _Extension)
     return wrapper
diff --git a/stix2/v20/sdo.py b/stix2/v20/sdo.py
index 430bfc0..b083e42 100644
--- a/stix2/v20/sdo.py
+++ b/stix2/v20/sdo.py
@@ -5,7 +5,6 @@ import itertools
 
 from stix2patterns.validator import run_validator
 
-from ..core import STIXDomainObject
 from ..custom import _custom_object_builder
 from ..exceptions import InvalidValueError
 from ..properties import (
@@ -14,6 +13,7 @@ from ..properties import (
     TimestampProperty, TypeProperty,
 )
 from ..utils import NOW
+from .base import STIXDomainObject
 from .common import ExternalReference, GranularMarking, KillChainPhase
 
 
@@ -374,5 +374,5 @@ def CustomObject(type='x-custom-type', properties=None):
             ],
             sorted([x for x in properties if x[0].startswith('x_')], key=lambda x: x[0]),
         ]))
-        return _custom_object_builder(cls, type, _properties, '2.0')
+        return _custom_object_builder(cls, type, _properties, '2.0', STIXDomainObject)
     return wrapper
diff --git a/stix2/v20/sro.py b/stix2/v20/sro.py
index b85eb68..0a07969 100644
--- a/stix2/v20/sro.py
+++ b/stix2/v20/sro.py
@@ -2,12 +2,12 @@
 
 from collections import OrderedDict
 
-from ..core import STIXRelationshipObject
 from ..properties import (
     BooleanProperty, IDProperty, IntegerProperty, ListProperty,
     ReferenceProperty, StringProperty, TimestampProperty, TypeProperty,
 )
 from ..utils import NOW
+from .base import STIXRelationshipObject
 from .common import ExternalReference, GranularMarking
 
 
diff --git a/stix2/v21/base.py b/stix2/v21/base.py
new file mode 100644
index 0000000..bfe1d5e
--- /dev/null
+++ b/stix2/v21/base.py
@@ -0,0 +1,24 @@
+"""Base classes for STIX 2.1 type definitions."""
+
+from ..base import _Extension, _Observable, _STIXBase
+from ..core import STIXDomainObject, STIXRelationshipObject
+
+
+class _STIXBase21(_STIXBase):
+    _spec_version = "2.1"
+
+
+class _Observable(_Observable, _STIXBase21):
+    pass
+
+
+class _Extension(_Extension, _STIXBase21):
+    pass
+
+
+class STIXDomainObject(STIXDomainObject, _STIXBase21):
+    pass
+
+
+class STIXRelationshipObject(STIXRelationshipObject, _STIXBase21):
+    pass
diff --git a/stix2/v21/bundle.py b/stix2/v21/bundle.py
index 168771f..ca60fe0 100644
--- a/stix2/v21/bundle.py
+++ b/stix2/v21/bundle.py
@@ -2,13 +2,13 @@
 
 from collections import OrderedDict
 
-from ..base import _STIXBase
 from ..properties import (
     IDProperty, ListProperty, STIXObjectProperty, TypeProperty,
 )
+from .base import _STIXBase21
 
 
-class Bundle(_STIXBase):
+class Bundle(_STIXBase21):
     # TODO: Add link
     """For more detailed information on this object's properties, see
     `the STIX 2.1 specification <link here>`__.
diff --git a/stix2/v21/common.py b/stix2/v21/common.py
index ac8daf1..4254767 100644
--- a/stix2/v21/common.py
+++ b/stix2/v21/common.py
@@ -2,7 +2,6 @@
 
 from collections import OrderedDict
 
-from ..base import _STIXBase
 from ..custom import _custom_marking_builder
 from ..exceptions import InvalidValueError
 from ..markings import _MarkingsMixin
@@ -13,9 +12,10 @@ from ..properties import (
     SelectorProperty, StringProperty, TimestampProperty, TypeProperty,
 )
 from ..utils import NOW, _get_dict
+from .base import _STIXBase21
 
 
-class ExternalReference(_STIXBase):
+class ExternalReference(_STIXBase21):
     # TODO: Add link
     """For more detailed information on this object's properties, see
     `the STIX 2.1 specification <link here>`__.
@@ -50,7 +50,7 @@ class ExternalReference(_STIXBase):
                 )
 
 
-class KillChainPhase(_STIXBase):
+class KillChainPhase(_STIXBase21):
     # TODO: Add link
     """For more detailed information on this object's properties, see
     `the STIX 2.1 specification <link here>`__.
@@ -62,7 +62,7 @@ class KillChainPhase(_STIXBase):
     ])
 
 
-class GranularMarking(_STIXBase):
+class GranularMarking(_STIXBase21):
     # TODO: Add link
     """For more detailed information on this object's properties, see
     `the STIX 2.1 specification <link here>`__.
@@ -79,7 +79,7 @@ class GranularMarking(_STIXBase):
         self._check_at_least_one_property(['lang', 'marking_ref'])
 
 
-class LanguageContent(_STIXBase):
+class LanguageContent(_STIXBase21):
     # TODO: Add link
     """For more detailed information on this object's properties, see
     `the STIX 2.1 specification <link here>`__.
@@ -107,7 +107,7 @@ class LanguageContent(_STIXBase):
     ])
 
 
-class TLPMarking(_STIXBase):
+class TLPMarking(_STIXBase21):
     # TODO: Add link
     """For more detailed information on this object's properties, see
     `the STIX 2.1 specification <link here>`__.
@@ -119,7 +119,7 @@ class TLPMarking(_STIXBase):
     ])
 
 
-class StatementMarking(_STIXBase):
+class StatementMarking(_STIXBase21):
     # TODO: Add link
     """For more detailed information on this object's properties, see
     `the STIX 2.1 specification <link here>`__.
@@ -150,7 +150,7 @@ class MarkingProperty(Property):
             raise ValueError("must be a Statement, TLP Marking or a registered marking.")
 
 
-class MarkingDefinition(_STIXBase, _MarkingsMixin):
+class MarkingDefinition(_STIXBase21, _MarkingsMixin):
     # TODO: Add link
     """For more detailed information on this object's properties, see
     `the STIX 2.1 specification <link here>`__.
@@ -215,7 +215,7 @@ def CustomMarking(type='x-custom-marking', properties=None):
 
     """
     def wrapper(cls):
-        return _custom_marking_builder(cls, type, properties, '2.1')
+        return _custom_marking_builder(cls, type, properties, '2.1', _STIXBase21)
     return wrapper
 
 
diff --git a/stix2/v21/observables.py b/stix2/v21/observables.py
index 8c9a2a1..e71e445 100644
--- a/stix2/v21/observables.py
+++ b/stix2/v21/observables.py
@@ -1,14 +1,13 @@
 """STIX 2.1 Cyber Observable Objects.
 
 Embedded observable object types, such as Email MIME Component, which is
-embedded in Email Message objects, inherit from ``_STIXBase`` instead of
-Observable and do not have a ``_type`` attribute.
+embedded in Email Message objects, inherit from ``_STIXBase21`` instead of
+_Observable and do not have a ``_type`` attribute.
 """
 
 from collections import OrderedDict
 import itertools
 
-from ..base import _Extension, _Observable, _STIXBase
 from ..custom import _custom_extension_builder, _custom_observable_builder
 from ..exceptions import AtLeastOnePropertyError, DependentPropertiesError
 from ..properties import (
@@ -18,6 +17,7 @@ from ..properties import (
     ObjectReferenceProperty, ReferenceProperty, StringProperty,
     TimestampProperty, TypeProperty,
 )
+from .base import _Extension, _Observable, _STIXBase21
 from .common import GranularMarking
 
 
@@ -142,7 +142,7 @@ class EmailAddress(_Observable):
     _id_contributing_properties = ["value"]
 
 
-class EmailMIMEComponent(_STIXBase):
+class EmailMIMEComponent(_STIXBase21):
     # TODO: Add link
     """For more detailed information on this object's properties, see
     `the STIX 2.1 specification <link here>`__.
@@ -214,7 +214,7 @@ class ArchiveExt(_Extension):
     ])
 
 
-class AlternateDataStream(_STIXBase):
+class AlternateDataStream(_STIXBase21):
     # TODO: Add link
     """For more detailed information on this object's properties, see
     `the STIX 2.1 specification <link here>`__.
@@ -271,7 +271,7 @@ class RasterImageExt(_Extension):
     ])
 
 
-class WindowsPEOptionalHeaderType(_STIXBase):
+class WindowsPEOptionalHeaderType(_STIXBase21):
     # TODO: Add link
     """For more detailed information on this object's properties, see
     `the STIX 2.1 specification <link here>`__.
@@ -316,7 +316,7 @@ class WindowsPEOptionalHeaderType(_STIXBase):
         self._check_at_least_one_property()
 
 
-class WindowsPESection(_STIXBase):
+class WindowsPESection(_STIXBase21):
     # TODO: Add link
     """For more detailed information on this object's properties, see
     `the STIX 2.1 specification <link here>`__.
@@ -841,7 +841,7 @@ class UserAccount(_Observable):
     _id_contributing_properties = ["account_type", "user_id", "account_login"]
 
 
-class WindowsRegistryValueType(_STIXBase):
+class WindowsRegistryValueType(_STIXBase21):
     # TODO: Add link
     """For more detailed information on this object's properties, see
     `the STIX 2.1 specification <link here>`__.
@@ -896,7 +896,7 @@ class WindowsRegistryKey(_Observable):
     _id_contributing_properties = ["key", "values"]
 
 
-class X509V3ExtenstionsType(_STIXBase):
+class X509V3ExtenstionsType(_STIXBase21):
     # TODO: Add link
     """For more detailed information on this object's properties, see
     `the STIX 2.1 specification <link here>`__.
@@ -988,7 +988,7 @@ def CustomObservable(type='x-custom-observable', properties=None, id_contrib_pro
             properties,
             [('extensions', ExtensionsProperty(spec_version='2.1', enclosing_type=type))],
         ]))
-        return _custom_observable_builder(cls, type, _properties, '2.1', id_contrib_props)
+        return _custom_observable_builder(cls, type, _properties, '2.1', _Observable, id_contrib_props)
     return wrapper
 
 
@@ -996,5 +996,5 @@ def CustomExtension(observable=None, type='x-custom-observable-ext', properties=
     """Decorator for custom extensions to STIX Cyber Observables.
     """
     def wrapper(cls):
-        return _custom_extension_builder(cls, observable, type, properties, '2.1')
+        return _custom_extension_builder(cls, observable, type, properties, '2.1', _Extension)
     return wrapper
diff --git a/stix2/v21/sdo.py b/stix2/v21/sdo.py
index f2dc0ea..42c58e4 100644
--- a/stix2/v21/sdo.py
+++ b/stix2/v21/sdo.py
@@ -7,7 +7,6 @@ import warnings
 from six.moves.urllib.parse import quote_plus
 from stix2patterns.validator import run_validator
 
-from ..core import STIXDomainObject
 from ..custom import _custom_object_builder
 from ..exceptions import (
     InvalidValueError, PropertyPresenceError, STIXDeprecationWarning,
@@ -18,6 +17,7 @@ from ..properties import (
     StringProperty, TimestampProperty, TypeProperty,
 )
 from ..utils import NOW
+from .base import STIXDomainObject
 from .common import ExternalReference, GranularMarking, KillChainPhase
 
 
@@ -828,6 +828,6 @@ def CustomObject(type='x-custom-type', properties=None):
             ],
             sorted([x for x in properties if x[0].startswith('x_')], key=lambda x: x[0]),
         ]))
-        return _custom_object_builder(cls, type, _properties, '2.1')
+        return _custom_object_builder(cls, type, _properties, '2.1', STIXDomainObject)
 
     return wrapper
diff --git a/stix2/v21/sro.py b/stix2/v21/sro.py
index 059bb66..91b8b80 100644
--- a/stix2/v21/sro.py
+++ b/stix2/v21/sro.py
@@ -2,12 +2,12 @@
 
 from collections import OrderedDict
 
-from ..core import STIXRelationshipObject
 from ..properties import (
     BooleanProperty, IDProperty, IntegerProperty, ListProperty,
     ReferenceProperty, StringProperty, TimestampProperty, TypeProperty,
 )
 from ..utils import NOW
+from .base import STIXRelationshipObject
 from .common import ExternalReference, GranularMarking