Lactacystin is an organic compound naturally synthesized by bacteria of the genus Streptomyces first described in 1991. The first total synthesis of lactacystin was developed by Elias Corey in 1992. The molecule is most commonly used as in biochemistry and cell biology laboratories as a selective inhibitor of the proteasome. The molecule is a lactam, or cyclic amide. A number of syntheses of this molecule have been published and there are no less than 1,300 references to this natural product in the literature.