To investigate the role of IDO1 in a mouse model of sJIA, we injected complete Freund’s adjuvant (CFA) in WT and IDO1-KO BALB/c mice and compared their disease courses with IFN-γ-KO mice, in which CFA-elicited symptoms are reminiscent of sJIA [22]. This evidence concerns the gene IFNG and systemic-onset juvenile idiopathic arthritis.