In situ adenoid cystic carcinoma (arrows) partially involving a duct (A; hematoxylin and eosin) associated with a primary adenoid cystic carcinoma of the breast (upper part of the micrograph). The partially involved duct contained nonneoplastic CK7-positive luminal cells (B), some of which were estrogen receptor–positive (C), and an in situ adenoid cystic carcinoma displaying MYB (D) and p63 (E) co-expression. The intact layer of myoepithelial cells surrounding the duct was highlighted by expression of p63 (E) and calponin (F). While p63 was also expressed in the in situ and invasive adenoid cystic carcinoma cells (E), c-KIT expression was observed in both the normal breast luminal epithelial cells and cells surrounding the pseudo-cystic structures of the in situ and invasive adenoid cystic carcinoma (G).