ESR1 and breast cancer: Among HR+/HER2- stage I-III (N = 466) and stage IV (N = 463) breast cancers, the most frequent alterations were PIK3CA mutations (42.5% vs 43.8%, respectively); followed by BRCA1/2 mutations (8.8% vs 7.3%), AKT1 (5.4% vs 3.9%), ESR1 (3.7% vs 3.9%), PTEN (2.6% vs 3.2%), PALB2 (1.5% vs 0.7%), and PD-L1 amplification (0.6% vs 0.7%).