The FDA approved the first ADC drug, a conjugate of an anti-CD33 monoclonal antibody and calicheamicin, Gemtuzumab ozogamicin (GO), for the treatment of patients aged > 60 years with relapsed CD33-positive acute myeloid leukemia (AML) in 2000 [91–93]. Here, CD33 is linked to acute myeloid leukemia.