Furthermore, alterations to SERT expression have been linked to reduced monoamine oxidase B (MAO-B) activity in ADHD; given MAO-B’s role in the catabolism of 5-HT, it may be possible that more efficacious uptake of 5-HT by SERT leads to this reduced activity [155]. This evidence concerns the gene SLC6A4 and attention deficit-hyperactivity disorder.