Abnormal Nuclear Membranous Staining Pattern by MLH1 Immunohistochemistry in Endometrial Cancer: A Diagnostic Pitfall and Clone-dependent Artifact.

Immunohistochemistry (IHC) for mismatch repair (MMR) proteins is routinely performed for endometrial cancer (EC). Loss of nuclear staining for MLH1/PMS2 triggers reflex testing for MLH1 promoter hypermethylation, while loss of MSH2/MSH6 or isolated loss of MSH6 and PMS2 prompts germline testing for Lynch syndrome. We observed an unusual nuclear membranous staining pattern of MLH1 (clone G168-15).