The human protein CGI-55 has been described as a chromo-helicase-DNA- binding domain protein (CHD)-S interacting protein and was also found to interact with the 3′-region of the plasminogen activator inhibitor (PAI)-1 mRNA. Here, we used CGI-55 as a "bait" in a yeast two-hybrid screen and identified eight interacting proteins: Daxx, Topoisomerase I binding RS (Topors), HPC2, UBA2, TDG, and protein inhibitor of activated STAT (signal transducer and activator of transcription) (PIAS)-1, -3, and -y These proteins are either structurally or functionally associated with promyelocytic leukemia nuclear bodies (PML-NBs), protein sumoylation, or the regulation of transcription. The interactions of CGI-55 with Daxx, Topors, PIASy and UBA2 were confirmed by in vivo co-localization experiments in HeLa cells, by using green (GFP) and red fluorescence fusion proteins. A mapping study of the CGI-55 binding site for these proteins revealed three distinct patterns of interaction. 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