ESR1 and breast cancer: Currently, tumor alterations in PIK3CA, AKT1, PTEN, ESR1, HER2, NTRK1/2/3, RET, and germline or somatic alterations in BRCA1/2 genes, germline PALB2 (gPALB2) alterations, as well as microsatellite instability (MSI) and high tumor mutation burden (TMB) represent actionable biomarkers in breast cancer.3