diff --git a/kythe/cxx/indexer/proto/marked_source.cc b/kythe/cxx/indexer/proto/marked_source.cc
index 1a5578cce2..325e8f856d 100644
--- a/kythe/cxx/indexer/proto/marked_source.cc
+++ b/kythe/cxx/indexer/proto/marked_source.cc
@@ -96,6 +96,22 @@ std::optional<MarkedSource> GenerateMarkedSourceForDescriptor(
     full_name = descriptor->full_name();
   }
   MarkedSource ms;
+  if (!descriptor->containing_oneof() && descriptor->label()) {
+    auto* mod = ms.add_child();
+    mod->set_kind(MarkedSource::MODIFIER);
+    switch (descriptor->label()) {
+      case google::protobuf::FieldDescriptor::Label::LABEL_OPTIONAL:
+        mod->set_pre_text("optional");
+        break;
+      case google::protobuf::FieldDescriptor::Label::LABEL_REQUIRED:
+        mod->set_pre_text("required");
+        break;
+      case google::protobuf::FieldDescriptor::Label::LABEL_REPEATED:
+        mod->set_pre_text("repeated");
+        break;
+    }
+    mod->set_post_text(" ");
+  }
   auto* type = ms.add_child();
   type->set_kind(MarkedSource::TYPE);
   switch (descriptor->type()) {
diff --git a/kythe/cxx/indexer/proto/testdata/basic/message-fields.proto b/kythe/cxx/indexer/proto/testdata/basic/message-fields.proto
index e4c26a45aa..11fe0fb0ec 100644
--- a/kythe/cxx/indexer/proto/testdata/basic/message-fields.proto
+++ b/kythe/cxx/indexer/proto/testdata/basic/message-fields.proto
@@ -16,7 +16,10 @@ message MessageFields {
   //- FieldNameNode.subkind field
   //- FieldNameNode typed Type1Node
   //- FieldNameNode code FNRoot
-  //- FNRoot child.1 FNName
+  //- FNRoot child.0 FNLabel
+  //- FNLabel.kind "MODIFIER"
+  //- FNLabel.pre_text "optional"
+  //- FNRoot child.2 FNName
   //- FNName child.0 FNQualName
   //- FNName child.1 FNIdent
   //- FNIdent.pre_text field_name
diff --git a/kythe/cxx/indexer/proto/testdata/basic/signatures.proto b/kythe/cxx/indexer/proto/testdata/basic/signatures.proto
index d58c9ff0ce..7333234910 100644
--- a/kythe/cxx/indexer/proto/testdata/basic/signatures.proto
+++ b/kythe/cxx/indexer/proto/testdata/basic/signatures.proto
@@ -10,22 +10,22 @@ package proto_kythe_test;
 message Message {
   //- @R defines/binding R
   //- R.code/rendered/qualified_name "proto_kythe_test.Message.R"
-  //- R.code/rendered/signature "string R"
+  //- R.code/rendered/signature "required string R"
   required string R = 1;
 
   //- @I defines/binding I
   //- I.code/rendered/qualified_name "proto_kythe_test.Message.I"
-  //- I.code/rendered/signature "int32 I"
+  //- I.code/rendered/signature "optional int32 I"
   optional int32 I = 2;
 
   //- @L defines/binding L
   //- L.code/rendered/qualified_name "proto_kythe_test.Message.L"
-  //- L.code/rendered/signature "Message L"
+  //- L.code/rendered/signature "repeated Message L"
   repeated Message L = 3;
 
   //- @#1E defines/binding E
   //- E.code/rendered/qualified_name "proto_kythe_test.Message.E"
-  //- E.code/rendered/signature "Enum E"
+  //- E.code/rendered/signature "optional Enum E"
   optional Enum E = 4;
 }