CD33 and acute lymphoblastic leukemia: Currently, there are four ADCs approved by regulatory agencies for cancer treatment, including gemtuzumab ozogamicin (anti-CD33 ADC) for acute myelogenous leukemia (AML), brentuximab vedotin (anti-CD30 ADC) for treating anaplastic large cell lymphoma/Hodgkin’s lymphoma, trastuzumab emtansine (Anti-HER2 ADC) for advanced HER2 (human epidermal growth factor receptor 2)-positive breast cancer, and inotuzumab ozogamicin (anti-CD22 ADC) for treatment of relapsed or refractory acute lymphoblastic leukemia (ALL) [13,14,15,16,17,18,19,20,21,22].