Cross-validation was used to internally validate CBCRisk-Mastectomy and compare with CBCRisk.<h4>Results</h4>CBCRisk-Mastectomy has nine predictors: first BC type, lobular carcinoma in situ status, estrogen receptor status, tumor stage, breast density, age at BC diagnosis, family history of BC, age at first birth, and body mass index. The gene discussed is ESR1; the disease is breast cancer.