ERBB2 and breast cancer: Five ADCs have been approved by the Food and Drug Administration: brentuximab vedotin for Hodgkin lymphoma [15], ado-trastuzumab emtansine for HER2-positive metastatic breast cancer [16, 17], inotuzumab ozogamicin for acute lymphoblastic leukemia [18], gemtuzumab ozogamicin for CD33-positive acute myeloid leukemia [19], and trastuzumab deruxtecan for unresectable or metastatic HER2-positive breast cancer patients who have received two or more prior anti-HER2-based regimens in a metastatic setting [20].