Proteasomes are large multicatalytic proteinase complexes located in the cytosol 
and the nucleus of eukaryotic cells. The ubiquitin-proteasome system is 
responsible for the degradation of most intracellular proteins and therefore 
plays an essential regulatory role in critical cellular processes including cell 
cycle progression, proliferation, differentiation, angiogenesis and apoptosis. 
Besides involving in normal cellular functions and homeostasis, the alteration 
of proteasomal activity contributes to the pathological states of several 
clinical disorders including inflammation, neurodegeneration and cancer. It has 
been reported that human cancer cells possess elevated level of proteasome 
activity and are more sensitive to proteasome inhibitors than normal cells, 
indicating that the inhibition of the ubiquitin-proteasome system could be used 
as a novel approach for cancer therapy. In this review we summarize several 
specific aspects of research for the proteasome complex, including the structure 
and catalytic activities of the proteasome, properties and mechanisms of action 
of various proteasome inhibitors, and finally the clinical development of 
proteasome inhibitors as novel anticancer agents.