Interestingly, cytokines that strongly correlate with kynurenine in children with NB in the present study (IL-4, IL-6, IL-8, IL-10 and IL-18) have been shown to be of importance for the induction of IDO1 either alone for IL-6 [29] and IL-18 [30] or together with IFN-γ and IL-4 [31], IL8 [32] and IL-10, [33,34]. Here, IL4 is linked to neuroblastoma.