Ki67 immunostaining of mammary tumors from WapCreC;Brca1;p53 double-mutant and WapCreC;Rank;Brca1;p53 triple-mutant females showed comparable proliferation of tumor cells in both genotypes (Supplementary information, Figure S9A and S9B), a finding that might explain that established tumors grow at a comparable “speed” (Supplementary information, Figure S5D). This evidence concerns the gene TP53 and breast cancer.