TPH2 and autism: Among them are genes encoding the serotonin transporter 5-HTT (transmission disequilibrium at the 5-HTT locus in 86 autism trios) (Cook et al., 1997), and a rate-limiting enzyme for serotonin synthesis TPH2 (two intronic SNPs rs4341581 and rs11179000 at introns 1 and 4, respectively, have been associated with autism) (Coon et al., 2005) (Table 2).