The five established molecular subtypes of breast cancer are luminal A, luminal B, ERBB2/HER2-overexpressing, basal-like breast cancer (BLBC), and normal-like tumors, based on the expression levels of estrogen receptor (ER), progesterone receptor (PR), Human Epidermal Growth Factor receptor 2 (HER2), and Ki-67. The gene discussed is ESR1; the disease is breast carcinoma.