RB1 and ductal breast carcinoma in situ: Functional inactivation of tumor suppressor p53 and retinoblastoma (Rb) by Tag lead to initiation of mammary hyperplasia around 8 weeks of age in hemizygous female mice in a hormone-independent manner that further develops to mammary intraepithelial neoplasia which resembles human ductal carcinoma in situ (DCIS) by around the 15th week of age22,23.