Although alterations in serum and urine levels of the tryptophan metabolite kynurenine have long been associated with disease (e.g. (15, 16), the specific contributions of IDO1 and IDO2 were first implicated in studies using the well-established KRN T cell receptor transgenic mouse model of arthritis (17) and the general IDO inhibitor 1MT (18). Here, IDO1 is linked to arthritic joint disease.