Another gene related to the serotonergic system is the brain-specific tryptophan hydroxylase-2 (TPH2), the rate-limiting enzyme in 5-HT synthesis in the brain, that was studied for the first time for its association in early-onset OCD (Mossner et al. 2006) and in ADHD by our groups (Walitza et al. 2005). The gene discussed is TPH2; the disease is attention deficit-hyperactivity disorder.