diff --git a/stix2/common.py b/stix2/common.py
index 602e43f..449cd54 100644
--- a/stix2/common.py
+++ b/stix2/common.py
@@ -3,7 +3,7 @@
 from collections import OrderedDict
 
 from .base import _STIXBase
-from .markings import MarkingsMixin
+from .markings import _MarkingsMixin
 from .properties import (HashesProperty, IDProperty, ListProperty, Property,
                          ReferenceProperty, SelectorProperty, StringProperty,
                          TimestampProperty, TypeProperty)
@@ -77,7 +77,7 @@ class MarkingProperty(Property):
             raise ValueError("must be a Statement, TLP Marking or a registered marking.")
 
 
-class MarkingDefinition(_STIXBase, MarkingsMixin):
+class MarkingDefinition(_STIXBase, _MarkingsMixin):
     _type = 'marking-definition'
     _properties = OrderedDict()
     _properties.update([
diff --git a/stix2/markings/__init__.py b/stix2/markings/__init__.py
index 7c45b5a..4038a5e 100644
--- a/stix2/markings/__init__.py
+++ b/stix2/markings/__init__.py
@@ -224,14 +224,14 @@ def is_marked(obj, marking=None, selectors=None, inherited=False, descendants=Fa
     return result
 
 
-class MarkingsMixin():
+class _MarkingsMixin():
     pass
 
 
 # Note that all of these methods will return a new object because of immutability
-MarkingsMixin.get_markings = get_markings
-MarkingsMixin.set_markings = set_markings
-MarkingsMixin.remove_markings = remove_markings
-MarkingsMixin.add_markings = add_markings
-MarkingsMixin.clear_markings = clear_markings
-MarkingsMixin.is_marked = is_marked
+_MarkingsMixin.get_markings = get_markings
+_MarkingsMixin.set_markings = set_markings
+_MarkingsMixin.remove_markings = remove_markings
+_MarkingsMixin.add_markings = add_markings
+_MarkingsMixin.clear_markings = clear_markings
+_MarkingsMixin.is_marked = is_marked
diff --git a/stix2/sdo.py b/stix2/sdo.py
index c3eb619..eaeef10 100644
--- a/stix2/sdo.py
+++ b/stix2/sdo.py
@@ -6,7 +6,7 @@ import stix2
 
 from .base import _STIXBase
 from .common import ExternalReference, GranularMarking, KillChainPhase
-from .markings import MarkingsMixin
+from .markings import _MarkingsMixin
 from .observables import ObservableProperty
 from .properties import (BooleanProperty, IDProperty, IntegerProperty,
                          ListProperty, PatternProperty, ReferenceProperty,
@@ -14,7 +14,7 @@ from .properties import (BooleanProperty, IDProperty, IntegerProperty,
 from .utils import NOW
 
 
-class STIXDomainObject(_STIXBase, MarkingsMixin):
+class STIXDomainObject(_STIXBase, _MarkingsMixin):
     pass
 
 
diff --git a/stix2/sro.py b/stix2/sro.py
index 4fa0465..60f99f5 100644
--- a/stix2/sro.py
+++ b/stix2/sro.py
@@ -4,14 +4,14 @@ from collections import OrderedDict
 
 from .base import _STIXBase
 from .common import ExternalReference, GranularMarking
-from .markings import MarkingsMixin
+from .markings import _MarkingsMixin
 from .properties import (BooleanProperty, IDProperty, IntegerProperty,
                          ListProperty, ReferenceProperty, StringProperty,
                          TimestampProperty, TypeProperty)
 from .utils import NOW
 
 
-class STIXRelationshipObject(_STIXBase, MarkingsMixin):
+class STIXRelationshipObject(_STIXBase, _MarkingsMixin):
     pass