SLC6A4 and autism: Positron emission tomography (PET) imaging studies have demonstrated abnormal 5‐HT synthesis in brain regions in autism (Chugani et al. 1997; Chugani et al. 2001; Chandana et al. 2005) and a polymorphism in the gene for the serotonin transporter (SLC6A4) has been associated with increased cerebral cortex grey matter volume in children with ASD (Wassink et al. 2007).