SLC6A4 and attention deficit-hyperactivity disorder: ADRA2A rs553668GG/GA and SLC6A4 rs6354 GG/GT genotypes display a 6.15-fold increase in the risk of ADHD in comparison to the cases carrying the combination of ADRA2A rs553668 AA and ANKK1 rs1800497 AA genotypes emphasizing the importance of the gene-gene interactions in the occurrence of the ADHD (Wang et al., 2021).