ERBB2 and neoplasm: After tumor progression to ET + CDK4/6i therapy, patients with HER2-low received a higher number of subsequent lines of systemic therapy (average in HER2-low: 1.47 ± 1.77; average in HER2-0: 1.24 ± 1.49, p < 0.0001) (Supplementary Table 1).