Hence, we conducted a systematic review and meta-analysis to comprehensively examine central and peripheral (free and total) TRP, the sum of CAAs and the TRP/CAAs ratio (index for the availability of TRP to the brain) along with indices of IDO, Kynurenine aminotransferase (KAT) and Kynurenine 3-monooxygenase (KMO) enzyme activities, and the levels of downstream neurotoxic TRYCATs in both MDD and BD in CNS, serum and plasma. This evidence concerns the gene KMO and major depressive disorder.