Currently, the vast majority of murine breast cancer cell lines used for inoculation into mice and genetically engineered mouse models (GEMMs) for breast cancer give rise to estrogen receptor–negative (ER−) mammary tumors (Özdemir et al. 2018), whereas ~75% of human invasive breast cancers are ER+ (Bentzon et al. 2007). The gene discussed is ESR1; the disease is invasive breast carcinoma.