BC is commonly categorized into four subtypes based on molecular characteristics and hormone receptor (HR) expression on immunohistochemistry (IHC): Human epidermal growth factor receptor 2 (HER2)-overexpression subtype (15–20% of BCs, ER+, PR+), Luminal A subtype (40–60% of BCs, ER+, PR+, Ki67 < 20%), Luminal B subtype (10–20% of BCs, ER+, PR+ or PR−, HER2+ or HER2−, Ki67 > 20%), and Triple-negative BC (TNBC)/basal-like subtype. The gene discussed is ESR1; the disease is breast cancer.