KRT5 and neoplasm: In Krt5-CreERT/Rosa26-mTmG /MMTV-PyMT tumors, co-expression of Krt5 and GFP was restricted to cells in the tumor periphery (Fig. 4I), suggesting that cells of the basal lineage were confined to the adjacent normal regions of the tumor, and that the small proportion of GFP-positive basal cells found in this tumor (Fig. 4C) are likely long-lasting myoepithelial cells present in adjacent normal tissues [35].