diff --git a/mutmut/__main__.py b/mutmut/__main__.py
index 24681bf4..77977085 100644
--- a/mutmut/__main__.py
+++ b/mutmut/__main__.py
@@ -738,6 +738,8 @@ def mangle_function_name(*, name, class_name):
     if class_name:
         assert CLASS_NAME_SEPARATOR not in class_name
         prefix = f'x{CLASS_NAME_SEPARATOR}{class_name}{CLASS_NAME_SEPARATOR}'
+    else:
+        prefix = 'x_'
     return f'{prefix}{name}'
 
 
@@ -752,6 +754,9 @@ def orig_function_and_class_names_from_key(mutant_name):
     if CLASS_NAME_SEPARATOR in r:
         class_name = r[r.index(CLASS_NAME_SEPARATOR) + 1: r.rindex(CLASS_NAME_SEPARATOR)]
         r = r[r.rindex(CLASS_NAME_SEPARATOR) + 1:]
+    else:
+        assert r.startswith('x_')
+        r = r[2:]
     return r, class_name
 
 
diff --git a/tests/test_mutation.py b/tests/test_mutation.py
index f7a8de29..a98e1a3a 100644
--- a/tests/test_mutation.py
+++ b/tests/test_mutation.py
@@ -194,11 +194,11 @@ def test_function_with_annotation():
     source = "def capitalize(s : str):\n    return s[0].upper() + s[1:] if s else s\n".strip()
     mutants = mutants_for_source(source)
     assert mutants == [
-        'def capitalize__mutmut_1(s : str):\n    return s[1].upper() + s[1:] if s else s',
-        'def capitalize__mutmut_2(s : str):\n    return s[None].upper() + s[1:] if s else s',
-        'def capitalize__mutmut_3(s : str):\n    return s[0].upper() - s[1:] if s else s',
-        'def capitalize__mutmut_4(s : str):\n    return s[0].upper() + s[2:] if s else s',
-        'def capitalize__mutmut_5(s : str):\n    return s[0].upper() + s[None] if s else s'
+        'def x_capitalize__mutmut_1(s : str):\n    return s[1].upper() + s[1:] if s else s',
+        'def x_capitalize__mutmut_2(s : str):\n    return s[None].upper() + s[1:] if s else s',
+        'def x_capitalize__mutmut_3(s : str):\n    return s[0].upper() - s[1:] if s else s',
+        'def x_capitalize__mutmut_4(s : str):\n    return s[0].upper() + s[2:] if s else s',
+        'def x_capitalize__mutmut_5(s : str):\n    return s[0].upper() + s[None] if s else s'
     ]
 
 
@@ -221,12 +221,12 @@ def foo():
 '''
     mutants = mutants_for_source(source)
     assert mutants == [
-        '\ndef foo__mutmut_1():    \n    dict(a=None, c=d)\n',
-        '\ndef foo__mutmut_2():    \n    dict(aXX=b, c=d)\n',
-        '\ndef foo__mutmut_3():    \n    dict(a=b, c=None)\n',
-        '\ndef foo__mutmut_4():    \n    dict(a=b, cXX=d)\n',
-        '\ndef foo__mutmut_5():    \n    dict( c=d)\n',
-        '\ndef foo__mutmut_6():    \n    dict(a=b,)\n',
+        '\ndef x_foo__mutmut_1():    \n    dict(a=None, c=d)\n',
+        '\ndef x_foo__mutmut_2():    \n    dict(aXX=b, c=d)\n',
+        '\ndef x_foo__mutmut_3():    \n    dict(a=b, c=None)\n',
+        '\ndef x_foo__mutmut_4():    \n    dict(a=b, cXX=d)\n',
+        '\ndef x_foo__mutmut_5():    \n    dict( c=d)\n',
+        '\ndef x_foo__mutmut_6():    \n    dict(a=b,)\n',
     ]
 
 
@@ -300,11 +300,11 @@ def foo():
 
 def test_orig_function_name_from_key():
     assert orig_function_and_class_names_from_key(f'_{CLASS_NAME_SEPARATOR}Foo{CLASS_NAME_SEPARATOR}bar__mutmut_1') == ('bar', 'Foo')
-    assert orig_function_and_class_names_from_key('bar__mutmut_1') == ('bar', None)
+    assert orig_function_and_class_names_from_key('x_bar__mutmut_1') == ('bar', None)
 
 
 def test_mangle_function_name():
-    assert mangle_function_name(name='bar', class_name=None) == 'bar'
+    assert mangle_function_name(name='bar', class_name=None) == 'x_bar'
     assert mangle_function_name(name='bar', class_name='Foo') == f'x{CLASS_NAME_SEPARATOR}Foo{CLASS_NAME_SEPARATOR}bar'
 
 
diff --git a/tests/test_mutmut3.py b/tests/test_mutmut3.py
index 89c70f06..411b9983 100644
--- a/tests/test_mutmut3.py
+++ b/tests/test_mutmut3.py
@@ -18,25 +18,25 @@ def foo(a, b, c):
 
 a + 1
 
-def foo__mutmut_orig(a, b, c):
+def x_foo__mutmut_orig(a, b, c):
     return a + b * c
 
-def foo__mutmut_1(a, b, c):
+def x_foo__mutmut_1(a, b, c):
     return a - b * c
 
-def foo__mutmut_2(a, b, c):
+def x_foo__mutmut_2(a, b, c):
     return a + b / c
 
-foo__mutmut_mutants = {
-'foo__mutmut_1': foo__mutmut_1, 
-    'foo__mutmut_2': foo__mutmut_2
+x_foo__mutmut_mutants = {
+'x_foo__mutmut_1': x_foo__mutmut_1, 
+    'x_foo__mutmut_2': x_foo__mutmut_2
 }
 
 def foo(*args, **kwargs):
-    return _mutmut_trampoline(foo__mutmut_orig, foo__mutmut_mutants, *args, **kwargs) 
+    return _mutmut_trampoline(x_foo__mutmut_orig, x_foo__mutmut_mutants, *args, **kwargs) 
 
-foo.__signature__ = _mutmut_signature(foo__mutmut_orig)
-foo__mutmut_orig.__name__ = 'foo'
+foo.__signature__ = _mutmut_signature(x_foo__mutmut_orig)
+x_foo__mutmut_orig.__name__ = 'x_foo'
 
 
 """
@@ -55,21 +55,21 @@ def foo(a: List[int]) -> int:
 
     expected = trampoline_impl + """
 
-def foo__mutmut_orig(a: List[int]) -> int:
+def x_foo__mutmut_orig(a: List[int]) -> int:
     return 1
 
-def foo__mutmut_1(a: List[int]) -> int:
+def x_foo__mutmut_1(a: List[int]) -> int:
     return 2
 
-foo__mutmut_mutants = {
-'foo__mutmut_1': foo__mutmut_1
+x_foo__mutmut_mutants = {
+'x_foo__mutmut_1': x_foo__mutmut_1
 }
 
 def foo(*args, **kwargs):
-    return _mutmut_trampoline(foo__mutmut_orig, foo__mutmut_mutants, *args, **kwargs) 
+    return _mutmut_trampoline(x_foo__mutmut_orig, x_foo__mutmut_mutants, *args, **kwargs) 
 
-foo.__signature__ = _mutmut_signature(foo__mutmut_orig)
-foo__mutmut_orig.__name__ = 'foo'
+foo.__signature__ = _mutmut_signature(x_foo__mutmut_orig)
+x_foo__mutmut_orig.__name__ = 'x_foo'
 
 
 """