In euthymic BD patients with PTSD, the KYN pathway might be less oriented toward the formation of the downstream metabolite QUIN, unlike what presumably occurs in bipolar depression: rather, there might prevalently be, in these subjects, a KYN biotransformation activating in parallel other enzymatic activities such as Kynureninase (Kynurenine hydrolase, KYNase), which might shift the pathway to other branches, along with only a moderate upregulation of main downstream enzymes as KMO. This evidence concerns the gene KMO and bipolar disorder.