Muc1 expression and gene targeting. (A) At E15.5, Muc1 (green) is expressed by the ductal core, in which Neurog3+ islet precursors (white, indicated by yellow arrowheads) appear to reside, as well as within the terminal elements of the epithelial network, adjacent to amylase-expressing acini (red). (B) While Muc1 remains strongly expressed throughout the adult ductal tree, Neurog3 is not detectable. (C) Muc1 (green) is not detected within or adjacent to adult islet β-cells (insulin, red) or α-cells (glucagon, white). Inset depicts Muc1 staining alone (white), with islet boundaries indicated by dashed line. (D) Structure of Muc1 locus, targeting vector and targeted allele. The targeting vector was designed to introduce an IRES-CreERT2 cassette downstream of the Muc1 stop codon, along with a FRT-flanked neoRconstruct for positive G418 selection and a tk gene at the end of each homology arm for negative FIAU selection in ES cells. Dotted lines indicate the boundaries of the 5' and 3' homology arms used for targeting, and the probe used for Southern blotting (outside the 5' homology arm) is indicated in red. Correct targeting introduces a new XbaI site, shifting the predicted restriction fragments detected by this probe. (E) Southern blotting of Muc1IC2neoES cells and mice. Xba1 digests of genomic DNA prepared from ES cells (parental R1 cells or targeted clone DK5.25), as well as from putative Muc1IC2neo/+ F1 offspring of DK5.25 chimeras (genotyped by PCR), were hybridized with the probe indicated in panel A. Targeted ES cells and mice display both wildtype (11 kb) and IC2neo (8.5 kb) XbaI fragments. Scale bars: 100 μm. Abbreviations: ac, acinus; du, duct; is, islet.