VSIR and monocytic leukemia: Collectively, these data suggest that PD-1H surface protein is highly expressed on AML blasts, but not on normal CD34+ progenitor cells; that PD-1H surface expression is higher in monocytic leukemia than in nonmonocytic leukemia and in monosomy or complex karyotype AML than in t(8;21) AML; and that high expression of PD-1H in AML BM results mainly from expression of PD-1H by AML blasts in addition to PD-1H expression on normal myeloid cells.