Lactacystin is a microbial metabolite isolated from Streptomyces that is now widely used as a selective inhibitor of the 20S proteasome. Clasto-lactacystin β-lactone was later identified as the active metabolite of lactacystin, resulting from the elimination of cysteine and the formation of a reactive β-lactone. Both lactacystin and its β-lactone metabolite induce differentiation and inhibit cell cycle progression in several tumor cell lines. Clasto-lactacystin β-lactone irreversibly alkylates subunit X of the 20S proteasome. It is at least 10 times more active than the parent compound; this increased activity may be a function of increased cell permeability. Inhibition of proteasome peptidase activity results in the accumulation of a variety of ubiquitinated proteins which would normally undergo rapid degradation. Thus, the effects of clasto-lactacystin β-lactone are pleiotropic and depend substantially on the expression pattern of signaling proteins within the treated cell.