ESR1 and cancer: In accordance with the standards set by the North American Registry of Cancer Centers, breast cancer subtypes are delineated as follows: estrogen receptor (ER)-positive/human epidermal growth factor receptor 2 (HER2)-negative breast cancer, HR-positive/HER2-positive breast cancer, HR-negative/HER2-positive breast cancer, and HR-negative/HER2-negative breast cancer (Harbeck and Gnant, 2017).