There are three major types of breast cancer: hormone receptor positive (HR+) breast cancer that expresses estrogen receptor (ER) with or without progesterone receptor (PR), human epithelial growth factor receptor 2 positive (HER2+) breast cancer, and triple-negative ( lack of estrogen receptor-α (ERα), progesterone receptor (PR), and human epidermal growth factor receptor-2 (HER2) amplification) breast cancer (TNBC). The gene discussed is ESR1; the disease is breast cancer.