ESR1 and breast cancer: Among HER2 + stage I-III breast cancers with early (N = 20) and late (N = 20) recurrence, the most frequent alteration, other than HER2 amplification, was PIK3CA (45% vs 25%), followed by BRCA1/2 (10% vs 5%), ESR1 (0% vs 5%), PD-L1 amplification (5% vs 0%).