KRT14 and neoplasm: To lineage trace Tgfbr2-deficient cells within these transitional SCC and enable isolation of specific Tgfbr2-deficient tumor cell populations, we backcrossed these mice into mice containing loxP sites flanking a STOP sequence preceding eYFP inserted into the Rosa26 locus (Figure 1—figure supplement 1), such that all K14-positive epithelial cells, including the anorectal SCC cells, while conditionally null for Tgfbr2 expressed YFP (cKO mice, Figure 1A–C).