CYP2C19 is a liver enzyme that acts on at least 10% of drugs in current clinical use (source: Genetics Home Reference2; see references), most notably the antiplatelet treatment clopidogrel (Plavix) but also drugs that treat pain associated with ulcers, such as omeprazole, antiseizure drugs such as mephenytoin, the antimalarial proguanil, and the anxiolytic diazepam. The gene discussed is CYP2C19; the disease is ulcer disease.