To recapitulate the phenotypic characteristics of tumors from ERα+ breast cancer patients harboring ESR1 mutations in a preclinical model, we employed ER+ MCF-7 cells that were genome edited using adeno-associated virus (AAV) technology to knock in the 2 most common ESR1 mutations, Y537S and D538G (13). The gene discussed is ESR1; the disease is breast carcinoma.