Subsequently, T-DXd was approved by the Food and Drug Administration (FDA) following the DestinyBreast-03trial, a phase 3 trial in which T-DXd showed a statistically significative improvement in progression-free survival (PFS) compared to T-DM1 in patients with HER2-positive mBC [28,29], becoming the gold standard in the second line of treatment of HER-2-positive BC. The gene discussed is ERBB2; the disease is breast cancer.