Leflunomide inhibits dihydroorotate dehydrogenase, preventing UMP formation and pyrimidine synthesis. Metabolites of leflunomide may inhibit various kinases such as JNK and JAK. Leflunomide is clinically used to treat rheumatoid arthritis; it exhibits immunosuppressive, antiviral, and anticancer activities. In several cellular models, leflunomide prevents apoptosis and increases viability in human cytomegalovirus (CMV)-infected cells and inhibits replication of polyomavirus BK. Additionally, leflunomide increases activity of aryl hydrocarbon receptors, inhibiting proliferation of melanoma cells.