Polymorphisms in the KCNIP1 (rs1541665) [64], SLC1A3 (rs1049522) [65], MTHFR (rs1801131) [66], ADRA2A (rs18005) [67] and SLC6A2 (rs5569) [68,69] genes were associated with ADHD risk, which is in line with previous reports. Here, MTHFR is linked to attention deficit-hyperactivity disorder.