A drug used to treat multiple myeloma. It is also used to treat mantle cell lymphoma in patients who have already received at least one other type of treatment and is being studied in the treatment of other types of cancer. PS-341 blocks several molecular pathways in a cell and may cause cancer cells to die. It is a type of proteasome inhibitor and a type of dipeptidyl boronic acid. Also called bortezomib and Velcade.