The MLL gene, also called HRX or ALL-1, was originally identified as a recurrent chromosomal translocation in particular subtypes of acute lymphocytic leukemia (ALL) and acute myelogenous leukemia (AML). Reciprocal rearrangements of the MLL gene are most common in infant ALL and secondary AML. Because of the unique association with infant leukemia and the intriguingly immature and mixed lineage phenotype of leukemic cells, the authors speculate that the wild-type MLL gene plays an important role early in the development of the hematopoietic system. This article reviews recent progress in understanding the function of the wild-type MLL protein, with particular consideration of potential functions within the developing hematopoietic system. Murine gain- and loss-of-function models have provided clues to the normal functions of MLL and altered functions of oncogenic MLL fusion proteins. Biochemical and genetic approaches using other model organisms have also elucidated mechanisms by which these functions are achieved.