The ubiquitin-proteasome system has become a promising molecular target in 
cancer therapy due to its critical role in cellular protein degradation, 
interaction with cell cycle and apoptosis regulation, and unique mechanism of 
action. Bortezomib (PS-341) is a potent and specific reversible proteasome 
inhibitor, which has shown strong in vitro antitumor activity as single agent 
and in combination with other cytotoxic drugs in a broad spectrum of 
hematological and solid malignancies. In preclinical studies, bortezomib induced 
apoptosis of malignant cells through the inhibition of NF-|B and stabilization 
of pro-apoptotic proteins. Bortezomib also promotes chemo- and 
radiosensitization of malignant cells in vitro and inhibits tumor growth in 
murine xenograft models. The proteasome has been established as a relevant 
target in hematologic malignancies and bortezomib has been approved for the 
treatment of multiple myeloma. This review summarizes recent data from clinical 
trials in solid tumors.