Structures of human polyubiquitin genes. UBB and UBC share genetic structures that consist of a promoter, a noncoding exon, an intron, and a Ub-coding exon. Trans-acting factors of UBB and UBC bind cis-acting elements located in the putative enhancer as well as in the promoter region. The putative enhancer can be found in the regulatory region including the intron. As an example of trans-acting factors, Sp1 and YY1 transcription factors binding to the UBC intron region are shown. Promotor and putative enhancer locations are displayed based on the candidate cis-acting regulatory elements (cCRE) from the Encyclopedia of DNA Elements (ENCODE) database. TSS, transcription start site.