ESR1 and neoplasm: Specifically, by determining the estrogen receptor (ER), progesterone receptor (PR), and HER2 status of a tumor, breast subtype can be approximated as follows: luminal A (ER+ or PR+ and HER2-), luminal B (ER+ or PR+ and HER2+), HER2+/ER- (ER- and PR- and HER2+), and triple-negative (ER- and PR- and HER2-) [13,14].