Pretreatment endoscopic biopsies stained with the MSH2 (A) and MSH6 (B) from one patient demonstrates intact nuclear immunopositivity in both neoplastic cells (glands) as well as background stromal cells (magnification, 40×). Sections from the same tumor in the post-treatment resection specimen stained with hematoxylin and eosin (C), Ki67 (D), MSH2 (E) and MSH6 (F), show scant residual dilated neoplastic glands, no expression of Ki-67 in the remaining neoplastic glands, retained expression of MSH2 in the neoplastic glands and background non-neoplastic cells (predominantly inflammatory cells and some fibroblasts), and rare neoplastic cells with weak expression of MSH6 and retained expression (with variable intensity) in many of the non-neoplastic cells (magnification, 40×).