In this phase I trial, for 58 HER2-positive breast cancer patients treated at 4.8 or 6.0 mg/kg (Table 3), the ORR was 70.7% (41/58; 95% CI, 57.3–81.9), and the DCR was 81.0% (47/58; 95% CI, 68.6–90.1), with all of the responding patients pretreated with trastuzumab ± pertuzumab, 39 responding patients (39/55, 70.9%) among those who had previously received at least one anti-HER2 TKI, 5 responding patients (5/12, 41.7%) among those who had previously received an anti-HER2 ADC, and 3 responding patients (3/8, 37.5%) among those who were pretreated with T-DM1. This evidence concerns the gene ERBB2 and breast cancer.