Muc1IRES-CreERT2 is active in adult exocrine cells. (A-C) Recombination was induced in adult (P60) Muc1IC2/+;Rosa26EYFP/+ mice by administration of 10 mg tamoxifen, and mice were analyzed 2 days later by immunofluorescence for EYFP (green), amylase (B, red) and cytokeratin-19 (C, red), together with DAPI (blue) to mark nuclei. After a short term chase, Muc1IC2 marks cells of the ductal network (closed arrowheads) as well as acinar cells (open arrowheads), suggesting that Muc1 is expressed in both duct and acinar cells. (D-E) Ductal Rosa26EYFPlabeling by Muc1IC2, shown in fields devoid of labeled acinar cells (D, magnified from panel C; E, independent field). As Rosa26EYFPexpression is lower in duct cells than acini, we have increased the green signal in these panels to make ductal staining more obvious. (F) 8-month old TM-untreated control mouse, stained for EYFP (green), amylase (red) and cytokeratin-19 (white). Less than 0.1% of duct and acinar cells are labeled in the absence of TM. Scale bars: 100 μm. Abbreviations: ac, acinus; du, duct; is, islet.