We found that 223 out of 1053 genes implicated in autism were regulated by 1,25-dihydroxyvitamin D. One of such ASD candidate genes was TPH2, coding for tryptophan hydroxylase 2, which is localized in neurons of the raphe nuclei and the enteric nervous system and is the enzyme responsible for producing all of the serotonin in the brain [30]. Here, TPH2 is linked to autism.