MAOA metabolizes stress-related neurotransmitters such as serotonin and norepinephrine [24], and SLC6A4 encodes the serotonin transporter [25]: over-exposure to stress-related neurotransmitters can have a significant impact on fetal development, synaptogenesis and neuronal cell division [26,27] and increase the risk for autism [28] and Attention Deficit /Hyperactivity Disorder (ADHD) [29]. Here, MAOA is linked to attention deficit-hyperactivity disorder.