We have previously established a mouse model carrying mammary-specific disruption of the full length of Brca1 (Brca1co/co;MMTV-Cre or Brca1co/co;Wap-Cre) (Brca1MKO), which develops mammary tumors through multiple progressive steps involving spontaneous mutations of p53 and/or some other unidentified genes5,14. This evidence concerns the gene BRCA1 and breast cancer.