Molyneux and colleagues developed a new method to determine the cell-of-origin for BRCA1-mutant breast cancer using a conditional mouse model of BRCA1 deficiency in which Cre recombinase-dependent deletion of exons encoding the BRCA1 protein's C-terminus, coupled with p53 heterozygosity, resulted in tumor development. The gene discussed is TP53; the disease is breast carcinoma.