The cellular proteasome is an important molecular target in cancer therapy and 
drug resistance research. Proteasome inhibitors are effective agents against 
multiple myeloma and mantle cell lymphoma and display great potential as 
treatment for a variety of other malignancies. The proteasome is a large 
multicatalytic, proteinase complex 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. Cancer cells are 
particularly sensitive to proteasome inhibitors, indicating the utility for 
inhibition of the ubiquitin-proteasome pathway as an approach for cancer 
therapy.