Case # 16: Carcinoma in situ with mixed pattern (CISM) showing dual cell population stained for hematoxylin and eosin, involving a papilloma and adjacent ducts. Areas of pleomorphic lobular pattern show discohesive atypical cells (arrows). Areas of “ductal” pattern show proliferation of cohesive cells with hyperchromatic nuclei forming secondary, rounded, and uniform lumens (arrowheads), (A, 200× and B, 400×). A mixed pattern resulted from a “collision” of both cell types in the same duct-lobular unit. “Ductal” cells are positive for E-cadherin (arrowheads) and lobular cells are negative (C and E– 200×, D and F – 400×).