Immunohistochemical (IHC) analysis of ER, PR, HER2, and Ki-67 (marker of proliferation Kiel 67) expression is used to categorize BC into four main molecular subtypes: Luminal A (HR+/HER2−), Luminal B (HR+/HER2−), HER2-enriched (HR−/HER2+), and basal-like [13,14,15]. The gene discussed is ESR1; the disease is breast cancer.