Differential expression of Hox genes is associated with normal hematopoiesis, whereas inappropriate maintenance of Hox gene expression, particularly Hoxa7 and Hoxa9, is a feature of leukemias harboring mixed-lineage leukemia (MLL) mutations. To understand the pathogenic roles of Hox genes in MLL leukemias, we assessed the impact of Hoxa7 or Hoxa9 nullizygosity on hematopoietic progenitor compartments and their susceptibility to MLL-induced leukemias. Selective reductions in the absolute numbers of committed progenitors, but not of hematopoietic stem cells, distinguished Hoxa7- and Hoxa9-deficient mice. Megakaryocytic/erythroid progenitor (MEP) reductions in Hoxa7^-/-mice correlated with reticulocytosis and thrombocytopenia without anemia. Conversely, Hoxa9^-/- mice displayed marked lymphopenia and substantial reductions of common lymphoid progenitors (CLPs) and lymphoid precursors, in addition to significant reductions of common myeloid progenitors (CMPs) and granulocyte/monocyte progenitors (GMPs). In retroviral transduction/transplantation assays, Hoxa7- and Hoxa9-deficient progenitors remained susceptible to transformation by MLL-GAS7, which activates MLL through a dimerization-dependent mechanism. However, Hoxa7^-/- or Hoxa9^-/-progenitors were less efficient in generating transformed blast colony-forming units (CFUs) in vitro and induced leukemias with longer disease latencies, reduced penetrance, and less mature phenotypes. Thus, Hoxa7 and Hoxa9 contribute to hematopoietic progenitor homeostasis but are not necessary for MLL-GAS7–mediated leukemogenesis, yet they appear to affect disease latency, penetrance, and phenotypes consistent with their critical roles as downstream targets of MLL fusion proteins. (Blood. 2004;103:3192-3199)