ESR1 and breast cancer: The breast cancer cohort of SUMMIT comprised heavily pretreated patients (n = 81) with metastatic ERBB2-mutant breast cancer who were treated with neratinib monotherapy (240 mg/day; n = 34, including 23 with ER+ breast cancer) or neratinib (240 mg/day) plus fulvestrant (500 mg on Days 1 and 15 of Month 1, then on Day 1 every 4 weeks; n = 47)79–96.