By immunofluorescence (IF) staining, we confirmed that both p53/BRCA1-deficient and C3(1)-Tag (but not MMTV-PyMT) mammary tumors contained many tumor cells expressing the basal marker, keratin 5 (K5), whereas all 3 tumor types were negative for ERɑ expression (S6 Fig). This evidence concerns the gene ESR1 and breast cancer.