The transcriptional repressor ZFM1 interacts with and modulates the ability of EWS to activate transcription.  The ZFM1 protein is both a transcriptional repressor and identical to the splicing factor SF1. ZFM1 was shown to interact with and repress transcription from the glycine, glutamine, serine, and threonine-rich transcription activation domain of the sea urchin transcription factor, stage-specific activator protein (SSAP). EWS, a human protein involved in cellular transformation in Ewing''s sarcoma tumors, contains an NH2-terminal transcriptional activation domain (NTD) which resembles that of SSAP in both amino acid composition and the ability to drive transcription to levels higher than VP16 in most cell types. Here we report that ZFM1 also interacts with EWS in both two-hybrid assays and glutathione S-transferase pull-down experiments. The region on EWS which interacts with ZFM1 maps to 37 amino acids within its NTD. Overexpression of ZFM1 in HepG2 cells represses the transactivation of reporter gene expression driven by Gal4-EWS-NTD fusion protein and this repression correlates with ZFM1 binding to EWS. Furthermore, two proteins, TLS and hTAFII68, which have extensive homology to EWS, also interact with ZFM1. Recently, it was discovered that EWS/TLS/hTAFII68 are each present in distinct TFIID populations and EWS and hTAFII68 were also found to be associated with the RNA polymerase II holoenzyme. The association of ZFM1 with these proteins implies that one normal cellular function for ZFM1 may be to negatively modulate transcription of target genes coordinated by these cofactors.