The major aims of the study are: to confirm the safety and tolerability of Hu5F9-G4 monotherapy in a relapsed/refractory AML and MDS population, and of Hu5F9-G4 in combination with azacitidine in previously untreated AML and MDS; and to evaluate the efficacy of Hu5F9-G4 monotherapy in relapsed/refractory AML/MDS, and of Hu5F9-G4 in combination with azacitidine in previously untreated AML/MDS, as measured by the objective response rate.