ESR1 and breast cancer: Notably, the CRC subtypes had obvious overlap with the known molecular subtypes of breast cancer defined by the expression of estrogen receptor (ER) or progesterone receptor (PR) and human epidermal growth factor receptor 2 (Her2) including ER/PR+, Her2+; ER/PR+, Her2−; ER/PR−, Her2+; and ER/PR−, Her2−.