TPH2 and attention deficit-hyperactivity disorder: Based on the finding, the TPH2 expression is decreased in carriers of the T allele [37], we expected to find a gene-dosage effect (GGTDC vs. T+TDC vs. GGADHD vs. T+ADHD), i.e. the WI phenotype is a direct result of the cumulative effect of WI phenotype (ADHD>TDC) and genetically moderated TPH2 variation (T+>GG).