Orlistat (also known as tetrahydrolipstatin, or Ro 18-0647, Lipase Inhibitor, THL ) is a potent and general lipase inhibitor with IC50 of 122 ng/ml for PL from human duodenal juice. Orlistat is a drug designed to treat obesity. Orlistat is the saturated derivative of lipstatin, a potent natural inhibitor of pancreatic lipases isolated from the bacterium Streptomyces toxytricini. Mechanistic studies suggest that Orlistat inhibits the thioesterase domain of fatty acid synthase (FAS) and as a result halts cell proliferation and causes cell apoptosis. More detailed experiments show that gradually Orlistat affects the retinoblastoma protein pathway and leads to cell-cycle arrest at G1/S boundary.