In addition to histological subtypes, immunohistochemical classification has divided BC into 5 main molecular subtypes: luminal A (estrogen receptor [ER] +, progesterone receptor [PR] +, human epidermal growth factor receptor 2 [HER2] –, Ki-67 low); luminal B HER2 – (ER+, PR+, HER2-, Ki-67 high); luminal B HER2+ (ER+, PR+, HER2+, Ki-67 high); HER2 (ER-, PR-, HER2+); and basal (triple negative [TNBC], ER-, PR-, HER2-), which are related to clinical outcome (5). The gene discussed is ESR1; the disease is breast cancer.