IDO2 and tricho-dento-osseous syndrome: Notably, we observed a significant increase of Kyn:Trp ratio, suggesting activation of Trp conversion into Kyn catalyzed either by inflammation-induced indoleamine-2,3-dioxygenase (IDO) or by stress-induced TDO [14], while successful metformin treatment of insulin resistance was reported recently to be associated with down-regulation of the Trp - Kyn pathway [32].