ESR1 and breast cancer: Currently, four major breast cancer subtypes can be distinguished based on the expressions of estrogen receptor (ER), progesterone receptor (PR), and Her2/Neu and are classified as the following in the order of increasing cancer aggressiveness: luminal A (ER+, PR+/−, and Her2−); luminal B (ER+, PR−/+, and Her2+); Her2-amplified (ER−, PR−, and Her2+); and triple negative or basal-like (ER−, PR−, and Her2−) (21).