HDC and Tourette syndrome: Moreover, the involvement of the brain histaminergic neurotransmission in the pathophysiology of Tourette syndrome, a condition commonly comorbid among ASD patients and featured by stereotypies, has been proposed and has been associated with a premature termination codon (W317X) responsible for the L-histidine decarboxylase (HDC) gene, the rate-limiting enzyme in biosynthesis of brain histamine.