Muc1IC2 labels cells throughout the ductal network. (A-C) Representative immunofluorescence images documenting the presence of Muc1IC2-labeled cells (green) within small/intercalated, medium/intralobular and large/interlobular ducts, co-stained for cytokeratin-19 (red). Interlobular were distinguished from intralobular ducts by the presence of connective tissue separating them from acinar parenchyma. Scale bar: 50 μm. (D) Muc1IC2 lineage-labeling indices of different duct populations, within various pulse-chase experiments. In each experiment, we observe roughly equal labeling of intercalated (blue circle), intralobular (red triangle) and interlobular (green X) ducts, with the exception of decreased intercalated duct labeling after tamoxifen treatment of adults (p < 0.01 by TukeyHSD).