Multiparous TgMFT121; TgWap-Cre mice remained tumor-free for more than a year after Cre-induction, but mice that were either heterozygous or homozygous for a conditional p53 allele [30] (TgMFT121; TgWAP-Cre; p53f/+ or TgMFT121; TgWAP-Cre; p53f/f) developed mammary tumors with 100% penetrance and clear evidence of p53 haploinsufficiency (Figure 1D, p<0.0001, log-rank test). This evidence concerns the gene TP53 and breast cancer.