The DESTINY-Breast03 trial, compared to T-DXd to T-DM1 in patients with metastatic HER2-positive breast cancer in the second-line setting, reported an impressive hazard ratio of 0.28 for disease progression or death (95% CI 0.22 to 0.35, p < 0.001) [66]. This evidence concerns the gene ERBB2 and breast cancer.