Notch1 expression impairs ductal side branching and stimulates luminal differentiation. (A) NOTCH1 inhibits mammary gland branching morphogenesis. Representative (n = 3) whole-mount analysis of mammary fat pads from wild-type, premalignant 3-month-old MMTV-tTA/Tet-Op-NOTCH1 or MMTV-tTA/Tet-Op-NOTCH1 mice treated with doxycycline for 7 days. Scale bars, 1,000 μm. (B) Constitutive NOTCH1 signaling promotes luminal differentiation. Matching fluorescence-activated cell sorting (FACS) histograms of mammary epithelial cells derived from the mice described in A. Cells were stained with antibodies against Ter119, CD45, and CD31, and the lineage-negative cells were then stained with CD29 and CD24 (top), and CD61 antibodies (bottom) and analyzed with flow cytometry. Dashed line, staining with an isotype-matched control.