ERBB2 and neoplasm: While our findings confirm and expand results of these studies in a larger cohort of patients receiving first-line CDK4/6i, they also provide first evidence that HER2-low status is associated with worse patient OS, which is unlikely to be attributable to the number of subsequent lines of therapy; indeed, in our study patients with HER2-low disease received a higher average number of subsequent lines of systemic therapy when compared to patients with HER2-0 disease after tumor progression to ET + CDK4/6 inhibitors.