Bortezomib (originally PS-341 and marketed as Velcade by Millennium Pharmaceuticals) is the first therapeutic proteasome inhibitor to be tested in humans. The boron atom within bortezomib catalytically binds the active site of the 26S proteasome with high affinity and specificity, thereby resulting in cell cycle arrest and apoptosis. In normal cells, the proteasome is involved in degradation of ubiquitylated proteins that have been tagged for destruction because they are damaged or unneeded by the cell. However, in cancerous cells, proteasome activity degrades pro-apoptotic proteins such as p53 that would normally result in programmed cell death of the dysfunctional cells. Proteasome inhibitors such as bortezomib interrupt this process, resulting in destruction of cancerous cells.