Though the mammary tumor incidence of p18−/− mice in the Balb/c-B6 mixed background was lower than those in the Balb/c background (47% vs. 83%) (Table 1 and Reference [19, 37]), p18−/− tumors in the Balb/c-B6 mixed background were also predominantly ERα-positive, Ck5/CK14 negative tumors (Table 1 and Additional file 1: Figure S1). Here, ESR1 is linked to breast cancer.