The partial tandem duplication of MLL (MLL-PTD) is found in 5% to 10% of patients with acute myeloid leukemia (AML) and normal cytogenetics. Its expression in leukemic blasts is coincident with a silenced wild-type (WT) MLL allele. We therefore generated mice expressing the Mll-PTD in the absence of Mll-WT. These Mll(PTD/-) mice die at birth unlike the normal life expectancy of Mll(PTD/WT), Mll(WT/-), and Mll(WT/WT) mice. Using Mll(WT/WT) fetal liver cells (FLC) as baseline, we compared Mll(PTD/-) with Mll(PTD/WT) FLC and found both had increased HoxA gene expression and granulocyte-macrophage colony-forming progenitor cells (CFU-GM); in contrast, only Mll(PTD/WT) FLC had increased pluripotent hemopoietic progenitors (CFU-GEMM). The similarities between Mll(PTD/WT) and Mll(PTD/-) mice suggest that the Mll-PTD mutation can up-regulate target genes in a dominant, gain-of-function fashion. The differences between these 2 genotypes suggest that in select tissues the Mll-PTD requires cooperation with the Mll-WT in the genesis of the observed abnormality.