Proteasome Inhibitor is a synthetic peptide (Z-Ile-Glu(OtBu)-Ala-Leu-CHO) which selectively inhibits chymotrypsin-like activities in the 26S proteasome (MCP; multicatalytic proteinase complex) in HT4 cells. Additionally, Proteasome Inhibitor acts as anti-neoplytic agent and induces apoptosis in different leukemia cell lines. Furthermore Proteasome Inhibitor is an inhibitor for the deubiquitinating Enzyme, and causes accumulation of ubiquitinylated proteins in neuronal cells. Proteasome Inhibitor prevents the activation of NF-κB in response to TNF-α or okadaic acid through inhibition of IκBα degradation.