Recently the COP9 signalosome (CSN) has become a focus of interest for many researchers, because of its function at the interface between signal transduction and ubiquitin-dependent proteolysis. It is required for the proper progression of the cell cycle in Schizosaccharomyces pombe and is essential for development in plants and Drosophila. However, its function in mammalian cells remains obscure. Although the CSN shares structural similarities with the 26S proteasome lid complex (LID), its functions seem to be different from that of the LID. A variety of CSN-specific protein-protein interactions have been described in mammalian cells. However, it is currently unclear how many reflect true functions of the complex. Two activities associated with the CSN have been identified so far: a protein kinase and a deneddylase. The CSN-associated kinase phosphorylates transcription factors, which determines their stability towards the ubiquitin system. The associated deneddylase regulates the activity of specific SCF E3 ubiquitin ligases. The CSN thus appears to be a platform connecting signalling with proteolysis.