IDO1 and in situ carcinoma: The kynurenines produced by Trp catabolism may be neuroprotective (kynurenic acid, picolinic acid) or neurotoxic (3‐hydroxykynurenine, quinolinic acid), with dysfunction suggested in MS by greater activity in pathways leading to production of neurotoxic mediators.22 With measurement of greater IDO1 mRNA in cells from patients with CIS, the present study suggests greater flux through the kynurenine pathway early in the MS disease course.