NEDD8 is a novel 81 amino acid polypeptide which is 60% identical and 80% homologous to ubiquitin(1,2). Ubiquitin is one of the most conserved eukaryotic proteins that can be conjugated to other proteins through a well defined enzymatic pathway. The importance of ubiquitination is underscored by its involvement in antigen processing, cell cycle regulation, degradation of tumor suppressors, receptor endocytosis, and signal transduction. This ubiquitin pathway involves three major enzymes and substrates. NEDD8 has been shown to activate and transfer to other proteins in a process analogous to ubiquitination.(3) The NEDD8 message was developmentally downregulated, and protein expression was highly enriched in the nucleus and weaker in the cytosol, whereas ubiquitin expression was detected equally well in the nucleus and cytosol(3). This antibody has been shown to detect a 6-kD NEDD8 monomer as well as a series of NEDD8 multimers (~14kD) or NEDD8-conjugated proteins (~90kD).