In addition, univariate tests showed differences between female SSc and HC participants in the interaction status for the TRP, IDO and TDO, and KYNA/TRP (TRP: F1,52 = 19.414, p < 0.001, η2p = 0.272; IDO and TDO: F1,52 = 14.284, p < 0.001, η2p = 0.215; KYNA/TRP: F1,52 = 11.420, p = 0.001, η2p = 0.180). The gene discussed is IDO1; the disease is systemic sclerosis.