Bortezomib, a boronic acid dipeptide, is a highly selective, reversible inhibitor of the 26S proteasome which primarily functions in the degradation of mis-folded proteins and is essential for the regulation of the cell cycle. Exposure to Bortezomib has been shown to stabilize p21, p27, and p53, as well as the proapoptotic Bid and Bax proteins, caveolin-1, and inhibitor κB-α, which prevents activation of nuclear factor κB-induced cell survival pathways. Bortezomib also promotes the activation of the proapoptotic c-Jun-NH2 terminal kinase, as well as the endoplasmic reticulum stress response. Alteration of the levels of these cellular proteins leads to inhibition of proliferation, migration, and promotion of apoptosis of cancer cells. Bortezomib is shown to penetrate into cells and inhibit proteasome-mediated intracellular proteolysis of long-lived proteins with a concentration that inhibits 50% of the proteolysis of ∼0.1 μM. The average growth inhibition of 50% value for Bortezomib across the entire panel of 60 cancer cell lines derived from multiple human tumors from the US National Cancer Institute (NCI) is 7 nM. Treatment of PC-3 cells with Bortezomib (100 nM) for 8 h results in the accumulation of cells in G2-M, with a corresponding decrease in the number of cells in G1. Bortezomib kills PC-3 cells at 24 and 48 hr with IC50 of 100 and 20 nM, respectively. Bortezomib induces nuclear condensation at 16–24 hr after treatment. Bortezomib treatment leads to PARP cleavage in a time-dependent manner with concentrations as low as 100 nM being effective at 24 hr.