While there have been further developments in anti-HER2 therapy, such as the introduction of a dual blockade with trastuzumab/pertuzumab or trastuzumab emtansin (T-DM1) as the first antibody–drug conjugate (ADC) for BC, all of these developments have followed the principle of therapy targeting HER2 overexpression, as the use of trastuzumab in tumors other than those that were HER2-positive did not show a clinical benefit. The gene discussed is ERBB2; the disease is breast cancer.