TPH2 and attention deficit-hyperactivity disorder: Furthermore, a meta-analysis of SNPs of TPH2 has shown a small but consistent association of SNPs in non-coding regions with one or more psychopathological condition, particularly with mood disorders, a common comorbidity of ADHD [51, 115] However, when looking at TPH2 variation in ADHD, results were mixed, with the meta-analysis reporting some studies finding associations but also highlighting one large European sample study where no associations between common TPH2 variants and ADHD were observed [113].