TP53 and breast cancer: In line with its tumorigenic potential in mouse embryonic fibroblasts (MEFs; Supplementary Figure 2) and previous studies showing the cooperation between Pik3caH1047R activating mutation and p53 knockout in vivo,17, 20 mice expressing one copy of myr-p110α and homozygous for p53 deletion (p53fl/fl;myr-p110αwt/fl) developed mammary tumors with 100% penetrance but with a long latency (mean tumor onset 259.6±10.6 days; Figure 1a).