IDO1, a rate-limiting enzyme in tryptophan metabolism, is located in the kynurenine production pathway and alteration of IDO1 expression changes the content of endogenous kynurenine and 5-HT, such that an increase in IDO1 activity lowers the endogenous 5-HT level (Hoglund et al., 2019), which leads to depression (Zhou et al., 2019) and diminished descending inhibition of pain modulation (Rojewska et al., 2018). This evidence concerns the gene IDO1 and depressive disorder.