NEDD8 belongs to the ubiquitin family and human NEDD8 shares 60% amino acid sequence identity to ubiquitin. The NEDD8 system is essential for the regulation of protein degradation pathways involved in cell cycle progression, morphogenesis and tumorigenesis. Attachment of NEDD8 to cullins activates their associated E3 ubiquitin ligase activity, and thus promotes polyubiquitination and proteasomal degradation of cyclins and other regulatory proteins.