IDO1 and chronic kidney disease: In light of the above discussion, the validity of KYN/TRP as an index of IDO1 activity needs reassessment in CKD and T2D context—a view that has recently been proposed by Badawy.41 It is relevant to acknowledge the potential involvement of indoleamine 2,3-dioxygenase 2 (IDO2) in TRP metabolism.42 IDO2 expression is predominant in the kidneys (as well as in antigen-presenting cells and a few other cell lines).43,44 While the biological role of IDO2 is yet to be fully elucidated, several studies point to IDO2 as an important contributor to TRP catabolism and production of KYN.