Aminopterin is a synthetic folic acid derivative whose metabolite is a competitive inhibitor of dihydrofolate reductase, which is a cofactor for nucleic acid synthesis. Aminopterin has anticancer and immunosuppressive properties. Formulations containing it have been used for pediatric leukemia though methotrexate is now more commonly used due to a more favorable therapeutic index. Aminopterin is a component of the widely used HAT (hypoxanthine-aminopterin-thymidine) medium to select cells for mammalian cell culture.