ESR1 and neoplasm: Pharmacotherapies (e.g., chemotherapy, endocrine therapy, and anti-human epidermal growth factor 2 ((HER2)) drugs) have been selected based on the subtypes of tumors according to the expression statuses of hormone receptors (estrogen receptor ((ER)), progesterone receptor ((PgR)) and HER2 because different tumor-growth pathways are activated in various subtypes of tumors [4].