ESR1 and breast cancer: Based on ER, PR, HER2, Ki-67 marker, breast cancer can be divided into 5 groups: Luminal A (ER+ or PR+; HER2-; Ki67 low), Luminal B HER2-negative (ER+ or PR+; HER2-; Ki67 high), Luminal B HER2-positive (ER+ or PR+; HER2+; Ki67 any), HER2-overexpression (ER-; PR-; HER2+; Ki67 any), Triple negative (ER-; PR-; HER2-; Ki67 any) (Onitilo et al., 2009).