TPH2 and attention deficit-hyperactivity disorder: Accumulating evidence has proposed that several functional polymorphisms of TPH2 gene are associated with psychiatric disorders including major depressive disorder (MDD), attention deficit hyperactivity disorder (ADHD), schizophrenia, and bipolar disorder (De Luca et al., 2004; De Luca et al., 2005; Sheehan et al., 2005; Cichon et al., 2007; Haghighi et al., 2008; Gao et al., 2012).