SLC6A4 and Anxiety: Serotonin re-uptake transporter genes, such as SERT-SLC6A4 and PMAT-SLC29A4, are of special interest for ASD for at least three reasons [8]: i) Serotonin has been shown to regulate the development of the central nervous system [9] and to be involved in a broad spectrum of behavioral and psychological processes (e.g., social behavior, aggression and anxiety) and various psychiatric disorders [10].