ESR1 and breast cancer: According to recent studies, BC can be classified into the following four subtypes: Luminal A (ER + /PR + /HER2 −, grade 1 or grade 2), Luminal B (ER + /PR + /HER2 + , or ER + /PR + /HER2 − grade 3), HER2-overexpressing (ER − /PR − /HER2 +), and triple-negative breast cancer (TNBC, ER − /PR − /HER2 −) [8].