TPH2 and depressive disorder: Several genetic polymorphisms have been analyzed to identify increased risk factors for developing depression in the course of IFN-α treatment, including serotonin transporter gene (5-HTTLPR) (Bull et al. 2009), 5-HT1A receptor gene (Kraus et al. 2007), tryptophan hydroxylase-2 gene (TPH2) (Kraus et al. 2007), phospholipase A2 (PLA2) and cyclooxygenase 2 (COX2) gene (Su et al. 2010).