ESR1 and breast cancer: Based on the genes expressed by tumor cells, breast cancers are divided into five subtypes: luminal A breast cancer [estrogen receptor (ER)-positive, progesterone receptor (PR)-positive, human epidermal growth factor receptor 2 (HER2)-negative, and low levels of Ki-67], luminal B breast cancer (ER-positive, HER2-negative, and high levels of Ki-67), luminal B-like breast cancer (ER-positive and HER2 positive), HER2-enriched breast cancer (ER-negative, PR-negative, and HER2-positive), and triple-negative breast cancer (ER-negative, PR-negative, and HER2-negative) [3, 4].