ESR1 and breast cancer: Driver gene alterations clinically relevant in breast cancer or related to CDK4/6i or ET biology were identified, including mutations in PIK3CA/AKT1/PTEN in 19 (44.2%) patients, BRCA1/BRCA2/PALB2 in 8 (18.6%) patients, FAT1 in 5 (11.6%) patients, TP53 in 4 (9.3%) patients, ESR1 in 3 (7.0%) patients (all previously exposed to AI), and ERBB2 in 2 (4.7%) patients.