IDO1 and amyotrophic lateral sclerosis: Since the study of Chen et al. included non-age-matched CONTR and ALS groups (average age of 36 and 58 years, respectively) and since the KP is involved in aging, these results, particularly with respect to CSF KYN [46], KYN/TRP ratios as a measure for IDO-activity, and concentrations of QA [47] could also be explained by the higher average age in the ALS group.