IDO1 and systemic sclerosis: In addition, univariate tests showed differences between male SSc and HC participants in the interaction status for the TRP, KYN, IDO and TDO, KYNA/TRP, and KATs (TRP: F1,43 = 13.715, p = 0.001, η2p = 0.242; KYN: F1,43 = 19.637, p < 0.001, η2p = 0.314; IDO and TDO: F1,43 = 50.391, p < 0.001, η2p = 0.540; KYNA/TRP: F1,43 = 6.418, p = 0.015, η2p = 0.130; KATs: F1,43 = 9.512, p = 0.004, η2p = 0.181).