Immunohistochemistry (IHC) receptors, i.e., estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2), are used to categorize BC tumors into four subtypes: luminal A (ER-positive/or PR-positive/HER2-negative), luminal B (ER-positive/or PR-positive/HER-positive), triple negative (ER-negative/PR-negative/HER-negative), and HER2-enriched (ER-negative/PR-negative/HER2-positive) (5). The gene discussed is ESR1; the disease is breast cancer.