MLH1 and colonic neoplasm: Immunohistochemistry (IHC) on surgical specimens was as follows: MLH1 stained negative and MSH2 stained positive in all tumors (Fig. 3), strongly suggesting a mutation in MLH1. Differential staining for cytokeratins 7 and 20 confirmed distinct histological origin of the ureteral and colon tumors (Fig. 3).