COP9 (Constitutive photomorphogenesis 9) signalosome (CSN) is a protein complex with isopeptidase activity. It catalyses the hydrolysis of NEDD8 protein from the cullin subunit of (CRL). Therefore, it is responsible for CRL – at the same time, it is able to bind denedyllated cullin-RING complex and retain them in deactivated form. COP9 signalosome thus serves as a sole deactivator of CRLs. The complex was originally identified in plants, and subsequently found in all eukaryotic organisms including human. Human COP9 signalosome (total size ~350 kDa) consists of 8 subunits - , CSN2, CSN3, CSN4, CSN5, CSN6, (COPS7A, COPS7B), CSN8. All are essential for full function of the complex and mutation in some of them is lethal in mice.