Four molecular subtypes of BC were categorized by gene expression sequence analysis; these include Luminal A, if estrogen receptor alpha-positive (ER) + and/or progesterone-receptor (PR) + , human epidermal growth factor receptor 2 (HER2) − , Ki67 < 14%), Luminal B (if ER + and/or PR + , HER2 overexpressed or Ki67 ≥ 14%), triple-negative breast cancer (TNBC) (if ER − , PR − , HER2 −), and HER2-enriched (if ER − , PR − and HER2 +) [2]. The gene discussed is ESR1; the disease is breast cancer.