We greatly appreciate Dr Jos Jonkers' lab for generously distributing mouse mammary tumour BRCA2 knockout cells (K14-Cre; Brca2F11/F11; p53F2-10/F2-10) and control mouse mammary tumour BRCA2 proficient cells (K14-Cre; Brca2wt/wt; p53F2-10/F2-10); Toshiyasu Taniguchi's lab for sharing C4-2 and PEO1 cells; Benjamin G. Neel's lab for sharing the 50 breast cancer cell lines; Ross D. Hannan, Geraldine Aubert, and Tehmina Masud for helpful suggestions to this paper. The gene discussed is BRCA2; the disease is breast carcinoma.