Moreover, future studies should a) focus on the probable causes of lowered TRP availability to the brain and unchanged IDO activity despite immune activation in mood disorders (including effects of nitrosative stress), and b) re-examine IDO activity and TRYCATs in association with staging of illness, which is largely mediated by immune and oxidative stress pathways (Maes et al., 2019b, 2022a). Here, IDO1 is linked to mood disorder.