Jun-dimerization protein 2 (JDP2) is a member of the activating protein-1 (AP-1) family of transcription factors. JDP2 dimerizes with other AP-1 proteins such as activating transcription factor-2 (ATF2) and Jun to repress transcription from promoters that contain a cyclic AMP-responsive element (CRE). Interferon regulatory factor-2-binding protein-1 (IRF2-BP1), which is reported to be a transcriptional corepressor of IRF2, was isolated as a JDP2-binding protein using an epitope-tagging method. As anticipated from the presence of a RING-finger domain, IRF2-BP1 enhanced the polyubiquitination of JDP2. Moreover, IRF2-BP1 repressed ATF2-mediated transcriptional activation from a CRE-containing promoter.