For example, targeting CD33 (an AML stem cell surface marker) with the FDA-approved drug gemtuzumab ozogamicin (Mylotarg), a recombinant humanized anti-CD33 monoclonal antibody conjugated to calicheamicin (a cytotoxic antibiotic), did produce some but low anti-leukemic activity in CD33+ AML patients 60 years and older who are not eligible for other cytotoxic therapies [34]. Here, CD33 is linked to acute myeloid leukemia.