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). The gene discussed is IDO1; the disease is major depressive disorder.