MMTV-Cre BRCA1 flox/flox p53+/- mice develop breast cancer with a latency of about 8 to 10 months, while MMTV-Cre BRCA1 flox/flox mice develop tumors with relatively low penetrance beyond age 1 year or older [34], and MMTV-Cre BRCA1 flox/wt mice rarely develop spontaneous breast cancers. This evidence concerns the gene BRCA1 and breast cancer.