It was FDA-approved in 1998 as an adjunct to cyclophosphamide, paclitaxel and/or doxorubicin in the treatment of early-stage HER2 positive breast cancer, and as a single drug for adjuvant treatment of early-stage, HER2 positive, high-risk ER/PR-negative breast cancers following multi-modality anthracycline-based therapy [10]. The gene discussed is ESR1; the disease is breast cancer.