High mobility group I proteins (HMGI, HMGY and HMGI-C) are a family of low molecular mass non-histone nuclear proteins which constitute an important component of the active chromatin structure. Two members of this family, HMGI and HMGY, have been demonstrated to contribute to the transcriptional regulation of several promoters by interacting with the DNA and with different transcription factors. On the contrary, very little is known about the third member, HMGI-C, which plays an important role during embryonic growth and in the process of cell transformation, its gene being rearranged in a large number of mesenchimal tumors. In this paper we show for the first time that HMGI-C is also able to function as architectural factor, enhancing the activity of a transcription factor, NF-κB, through the PRDII element of the β-interferon enhancer. Moreover we show that this enhancement is absolutely dependent on the binding of HMGI-C to its target sequence. The demonstration that HMGI-C is able to modulate transcription is thus an important initial step in the identification of genes regulated by this factor.