SLC6A4 and attention deficit-hyperactivity disorder: Among these SNPs, ADRA2A rs553668G allele was associated with an increased risk of ADHD in the additive model, and SLC6A4 rs6354 (GG + GT) was associated with an increased risk of ADHD, whereas DRD2 rs1124491 (AA) was associated with a decreased risk of ADHD.