Lamhonwah et al. [32] recently reported an association between a deletion in the SLC22A5 gene with attention-deficit/hyperactivity disorder, and Celestino-Soper et al. [33,34] identified abnormal L-carnitine biosynthesis in several patients with autism spectrum disorders (ASD), suggesting that L-carnitine deficiency could represent a risk factor for autism and supporting the potential, beneficial effects of L-carnitine supplementation in these patients [35]. This evidence concerns the gene SLC22A5 and attention deficit-hyperactivity disorder.