The COP9 signalosome (CSN) is an evolutionarily conserved protein complex of the eight subunits that interacts with deubiquitinating enzymes and protein kinases and is highly homologous to the lid sub-complex of 26S proteasome. The CSN complex is an essential regulator of the ubiquitin conjugation pathway by mediating the deneddylation of the SCF-type E3 ligase complexes, which leads to a decrease in ubiquitin ligase activity of SCF-comlpexes such as SCF, CSA or DDB2.  It is also involved in phosphorylation of p53, c-jun/JUN, ITPK1 and IRF8/ICSBP, possibly via its association with CK2 and PKD kinases. CSN8 encodes the smallest and the least conserved but first identified subunit of CSN. Recent studies show CSN8 is essential for Drosophila development and is essential for peripheral T cell homeostasis and antigen receptor-induced entry into the cell cycle from quiescence.