IDO1 and osteoporosis: In osteoporosis, d-tryptophan does not engage in the KYN/AhR/RANKL pathway, as indoleamine 2,3-dioxygenase 1 (IDO1) cannot process the d-form of tryptophan; additionally, its effects on the IDO1/KYN/AhR pathway are secondary to its primary action on the IFN-γ/mini-TrpRS axis during the early stages of osteoclastogenesis (Kim et al., 2012).