Given the effectiveness of proteasome inhibition in multiple myeloma, the role of proteasome in solid tumors is under active investigation. Previous trials of bortezomib in breast, prostate, lung, and pancreatic cancer have shown little activity of this agent in these diseases. Whether the lack of activity may be mechanistically related (bortezomib inhibits chymotrypsin-like and peptidyl-glutamyl peptide-hydrolyzing (PGPH)-like activities of the proteasome), or a lack of potency in target inhibition, is unknown.