We found that 47% of p18−/−, 73% of p18−/−;Brca1MGKO, 60% of p16−/−;Brca1MGKO, and 8% of Brca1MGKO mice in the Balb/c-B6 mixed background developed mammary tumors, yet no p16−/− mice developed mammary tumors at similar ages (Table 1 and Additional file 1: Figure S1). The gene discussed is CDKN2A; the disease is breast cancer.