The c-myc intron binding protein 1 (MIBP1) is a gigantic zinc finger protein comprising 2,437 amino acids and belonging to the MHC binding protein (MBP) family. MIBP1 is suggested to be a transcription factor involved in various biological functions. We show here that MIBP1 represses c-myc transcription from the major promoter, P2. Screening by the yeast two-hybrid system revealed that the MIBP1 protein interacts with the Ski-interacting protein (SKIP). In vitro pull-down assays and in vivo co-immunoprecipitation experiments confirmed this interaction. The acidic region of MIBP1 was found to be the site of interaction with the N-terminal half of SKIP. In situ hybridization analysis using developing rat embryos revealed that the MIBP1 mRNA is highly expressed in post-mitotic neurons, but the expression in immature neuroepithelium is low. The expression of MIBP1 in adult rat brain is also predominantly in neuronal cells, indicating that MIBP1 is involved in the physiology of mature neuronal cells.