Deregulated NOTCH1 expression results in mammary gland transformation. (A) Constitutive NOTCH1 signaling results in mammary tumor formation. A cohort of wild-type (n = 15) and MMTV-tTA/TOP-ICN1 (n = 65) mice were maintained under mating conditions and monitored for tumor formation. The proportion of tumor-free mice was plotted by using the Kaplan-Meier software. (B) NOTCH1 expression leads to tubular adenocarcinomas. Sections from MMTV-tTA/TOP-ICN1 primary mammary tumors were fixed in 10% formalin, paraffin embedded, sectioned, and stained with hematoxylin and eosin (H&E). Analysis of tumors showed multiple initiations within a single fat pad, with cordlike structures of two or three cell layers separated by stroma. Scale bars, 100 μm. (C) NOTCH1-transformed mammary tumors are ERα-negative and express the luminal lineage cytokeratins 8 and 18. Tumor sections from wild-type mammary fat pad (left) or primary tumor (right) were stained with antibodies against ERα (top), Cytokeratin 8/18 and Cytokeratin 5 (middle), or Cytokeratin 14, and counterstained with DAPI (bottom). Scale bars, 100 μm.