ESR1 and breast cancer: Among HER2 + stage I-III (N = 95) and stage IV (N = 78) breast cancers, the most frequent alterations, other than HER2 amplification, were PIK3CA mutations (26% vs 41.0%), followed by BRCA1/2 (6.3% vs 6.4%), ESR1 (1.1% vs 3.9%) mutations, and PD-L1 amplification (1.1% vs 0%).