diff --git a/rules/aip0121/no_mutable_cycles.go b/rules/aip0121/no_mutable_cycles.go
index 51d19ae1a..3d87500a2 100644
--- a/rules/aip0121/no_mutable_cycles.go
+++ b/rules/aip0121/no_mutable_cycles.go
@@ -51,6 +51,10 @@ func findCycles(start string, node *desc.MessageDescriptor, seen stringset.Set,
 			continue
 		}
 		if ref.GetType() == start {
+			// Allow for mutable self-reference to support taxonomy cases.
+			if len(chain) == 1 {
+				continue
+			}
 			cycle := strings.Join(append(chain, start), " > ")
 			problems = append(problems, lint.Problem{
 				Message:    "mutable resource reference introduces a reference cycle:\n" + cycle,
diff --git a/rules/aip0121/no_mutable_cycles_test.go b/rules/aip0121/no_mutable_cycles_test.go
index 2d055008a..70dcb2b09 100644
--- a/rules/aip0121/no_mutable_cycles_test.go
+++ b/rules/aip0121/no_mutable_cycles_test.go
@@ -36,19 +36,17 @@ func TestNoMutableCycles(t *testing.T) {
 			nil,
 		},
 		{
-			"InvalidCycle",
+			"ValidSelfReferenceCycle",
+			"",
 			`[(google.api.resource_reference).type = "library.googleapis.com/Publisher"]`,
-			`[(google.api.resource_reference).type = "library.googleapis.com/Book"]`,
 			"",
 			"",
-			testutils.Problems{{
-				Message: "cycle",
-			}},
+			nil,
 		},
 		{
-			"InvalidSelfReferenceCycle",
-			"",
+			"InvalidCycle",
 			`[(google.api.resource_reference).type = "library.googleapis.com/Publisher"]`,
+			`[(google.api.resource_reference).type = "library.googleapis.com/Book"]`,
 			"",
 			"",
 			testutils.Problems{{
@@ -65,21 +63,6 @@ func TestNoMutableCycles(t *testing.T) {
 				Message: "cycle",
 			}},
 		},
-		{
-			"InvalidDeepAndShallowCycles",
-			`[(google.api.resource_reference).type = "library.googleapis.com/Publisher"]`,
-			`[(google.api.resource_reference).type = "library.googleapis.com/Library"]`,
-			`[(google.api.resource_reference).type = "library.googleapis.com/Book"]`,
-			`[(google.api.resource_reference).type = "library.googleapis.com/Book"]`,
-			testutils.Problems{
-				{
-					Message: "cycle",
-				},
-				{
-					Message: "cycle",
-				},
-			},
-		},
 		{
 			"ValidOutputOnlyCyclicReference",
 			`[(google.api.resource_reference).type = "library.googleapis.com/Publisher"]`,