ESR1 and breast cancer: Several key prognostic factors for BC include the expression of estrogen receptor α (ERα), which distinguishes tumors as ERα-positive or ERα-negative, the status of breast cancer type 1 susceptibility protein (BRCA1) (wild type—wt versus mutated), and the expression of progesterone receptor (PR) and HER2, further dividing different subgroups within the LumA and LumB phenotypes.