Since HER3 is mutated in up to 14% of metastatic ER+ breast cancers [16], we introduced the above HER3 mutations along with HER3EV (EV- empty vector) and HER3WT (WT- wild-type) into ER+ T47D and MCF-7 cells using lentiviral transduction as described in Materials and Methods. The gene discussed is ESR1; the disease is breast cancer.