Table 1 summarizes the rates of cardiomyopathy in studies assessing the use of T-DXd for the treatment of patients with breast cancer. The rates of decreased EF are similar to the ones reported for trastuzumab trials [42]. Although the patient numbers are small, in DESTINY-Breast03, one patient treated with T-DM1 was found to have a decreased EF and six in the T-DXd arm; more information is needed to determine if there are differences in safety between the HER2-directed ADCs [29]. This evidence concerns the gene ERBB2 and breast cancer.