A recent study analyzing the occurrence of adverse drug reactions associated with HER2-positive BC through a spontaneous reporting system database showed that half of the adverse reactions associated with T-DM1 included blood disorders, such as thrombocytopenia, while cardiac disorders, including heart failure and arrhythmias, were significantly less frequent compared to their incidence associated with classical anti-HER2 antibodies (trastuzumab and pertuzumab), with an overall incidence of only 3% (93). The gene discussed is ERBB2; the disease is breast cancer.