BRCA1 and breast cancer: All intermediate-grade tumors that developed in double- and triple-mutant mice expressed the basal epithelial marker CK5; however, in high-grade tumors from WapCreC;Brca1;p53 double-mutant females we observed marked downregulation of CK5 expression (Figure 2B, Supplementary information, Figure S7A and S7B), supporting the notion of manifest epithelial dedifferentiation of these high-grade mammary cancers.