In fact, the very first ADC to gain marketing approval by the US Food and Drug Administration (FDA) was gemtuzumab ozogamicin, consisting of a humanized anti-CD33 monoclonal antibody conjugated to the DNA-damaging agent calicheamicin, for the treatment of acute myeloid leukemia (AML) [21]. The gene discussed is CD33; the disease is acute myeloid leukemia.