Currently, 14 ADCs are approved by the Food and Drug Administration (FDA), and an additional 198 are in clinical trials.8, 9, 10 Recently, ADC use has expanded to earlier stages and a wider range of tumor types.11, 12, 13 For example, trastuzumab deruxtecan (Enhertu), an ADC targeting the human epidermal growth factor receptor (HER)2 receptor, was first approved by the FDA in 2019 for the treatment of unresectable or metastatic HER2+ breast cancer (after patients have received 2 or more anti-HER2 therapies).14 This evidence concerns the gene ERBB2 and breast cancer.