[17]–[19] For example, we recently reported that AhR activation by leflunomide, a well known immunosuppressive agent used to treat rheumatoid arthritis, alters cell proliferation and tissue regeneration in a context-specific manner. [18], [20] Leflunomide is converted to its primary metabolite A771726 via isoxazole ring cleavage, and whereas metabolism of leflunomide to A771726 is required for dihydroorotatedehydrogenase (DHODH) inhibition, [21] this conversion significantly abrogates the AhR-activating properties of leflunomide. [18]. Here, AHR is linked to rheumatoid arthritis.