Increased microglial degradation of tryptophan, supported by changes in IDO isoforms and QUIN levels, could help explain the decreased levels of serotonin metabolites measured in the CSF of individuals with MDD and SB (Brown et al., 1982; Mann et al., 1992; Steiner et al., 2011; Bakunina et al., 2015; Fujigaki et al., 2017; Figure 1). The gene discussed is IDO1; the disease is major depressive disorder.