In 1971, Akira Endo, a Japanese biochemist working for the pharmaceutical company Sankyo, identified mevastatin (ML-236B), a molecule produced by the fungus Penicillium citrinum, as an inhibitor of HMG-CoA reductase, a critical enzyme used by the body to produce cholesterol. Animal trials showed very good inhibitory effect as in clinical trials, however a long term study in dogs found toxic effects at higher doses and as a result mevastatin was believed to be too toxic for human use. Mevastatin was never marketed, because of its adverse effects of tumors, muscle deterioration, and sometimes death in laboratory dogs.