CD33 and acute promyelocytic leukemia: GO is an anti-CD33 monoclonal antibody conjugated to the toxin calicheamicin, and has shown significant activity in APL because of the high level of expression of CD33 target antigen on APL cells.97 However, safety concerns led to the US marketing withdrawal of GO in June 2010, although this decision has more recently been called into question.98, 99 GO is currently available under compassionate use programs.