The frequent occurrence of p53 mutations in ovarian cancers in women with BRCA1 mutations [25], [26] and the apparent cooperation of Brca1 and p53 in mouse models of mammary cancers [27]–[31] led us to perform experiments in which conditional inactivation of Brca1 was targeted to the OSE, along with the conditional inactivation the tumor suppressors p53 and/or Rb. Our results show that invasive tumors developed in all mice that had inactivation of p53 targeted to the OSE and that tumor development was accelerated in mice with concomitant inactivation of Brca1. The gene discussed is BRCA1; the disease is breast cancer.