The (11;19)(q23;p13.1) translocation in acute leukemia leads to the generation of a chimeric protein that fuses MLL to the transcriptional elongation factor ELL. A novel protein was isolated from a yeast 2-hybrid screen with ELL that was named EAF1 for ELL-associated factor 1. Using specific antibodies, the endogenous EAF1 and ELL proteins were coimmunoprecipitated from multiple cell lines. In addition, endogenous EAF1 also exhibited the capacity to interact with ELL2. Database comparisons with EAF1 identified a region with a high content of serine, aspartic acid, and glutamic acid residues that exhibited homology with the transcriptional activation domains of several translocation partner proteins of MLL, including AF4, LAF4, and AF5q31. A similar transcriptional activation domain has been identified in this region of EAF1. By confocal microscopy, endogenous EAF1 and ELL colocalized in a distinct nuclear speckled pattern. Transfection of the MLL-ELL fusion gene delocalized EAF1 from its nuclear speckled distribution to a diffuse nucleoplasmic pattern. In leukemic cell lines derived from mice transplanted with MLL-ELL-transduced bone marrow, EAF1 speckles were not detected. Taken together, these data suggest that expression of the MLL-ELL fusion protein may have a dominant effect on the normal protein-protein interactions of ELL.