The COP9 signalosome (CSN) is a multiprotein complex that was initially 
identified in plants as a repressor of photomorphogenesis. It is now known to 
play major roles in several other developmental pathways, from auxin response to 
flower development. Furthermore, the COP9 signalosome shares homologies with the 
lid sibcomplex of the proteasome and is evolutionarily conserved from fission 
yeast to humans. It is important for the proper development of virtually all 
higher eukaryotes. In recent years, significant progress has been made in 
unraveling the molecular, cellular, and physiological mode of action of the COP9 
signalosome. This review discusses our current understanding of the COP9 
signalosome function with particular emphasis on its recently defined role in 
modulating a wide variety of cellular processes by regulating specific protein 
degradation events.