The ELL gene was first identified by its involvement with MLL in the translocation (11;19)(q23;p13.1) in acute myeloid leukemia. To date, nine other MLL partner genes have been cloned, but their precise functions have yet to be determined. To characterize the functions of ELL further, we have cloned the murine homologue of ELL and have found that the gene is highly conserved at the nucleotide and amino acid level. The open reading frame of the murine homologue contains 602 aa, slightly smaller than the 621 aa in the human gene. With Northern blot analysis, a 3.4-kb transcript is detected in all tissues examined with greatest levels of expression in the liver. Unlike human ELL, only a single transcript can be detected with either murine coding sequence or 3' untranslated region probes. To examine the spatial and temporal pattern of expression in murine development, in situ hybridization studies were performed with sense and antisense riboprobes from the 3' untranslated region of murine Ell. Ell is expressed diffusely by embryonic day 7.5 (E7.5). In addition, high levels of expression can be detected in maternally derived decidual tissue. At E14.5, Ell is expressed diffusely throughout the embryo. However by E16.5, specific expression in the liver and gastrointestinal tract becomes prominent and remains so in both neonates and adults. To determine the subcellular localization of ELL, we developed a polyclonal antiserum to ELL that was used for immunofluorescence studies in COS-7, HeLa, NIH 3T3, and A7r5 cells. The ELL protein was localized to the nucleus but excluded from nucleoli in all cell lines examined. Recently, the gene product of ELL was found to function as an RNA polymerase II elongation factor, an activity that is consistent with our immunofluorescence data. Thus, these studies extend our understanding of the normal functions of ELL and provide additional insight into its aberrant function when fused to MLL in acute myeloid leukemia.