The widely adopted method for clinical classification of breast cancer subtypes is usually based on immunohistochemical (IHC) analysis of estrogen receptor (ER), progesterone receptor (PR) and human epidermal receptor 2 (HER2), categorizing breast carcinomas into hormone receptor positive and HER2 negative (ER + and/or PR + and HER2-), hormone receptor positive and HER2+ (ER + and/or PR + and HER2+), hormone receptor negative and HER2 positive (ER-/PR-/HER2+) and triple negative breast cancer (TNBC, ER-/PR-/HER2-) subtypes. The gene discussed is ESR1; the disease is breast cancer.