Transplantation of ER-positive p18−/− tumor cells into NSG mice did not produce tumors, whereas, with exogenous E2 supplement, p18−/− tumor cells generated ER-positive tumors in NSG mice, suggesting that host estrogen is not sufficient to induce p18 deficient, Brca1-proficient tumor initiation. Here, CDKN2C is linked to neoplasm.