Alteration of the brain’s HA modulatory system has been identified as being a rare genetic cause of tic disorders; indeed, histidine decarboxylase knockout (Hdc-KO) mice have been generated, representing a promising model of this pathophysiology [38,39], implying that diminished histaminergic neurotransmission may be related to the exhibited repetitive and tic-like stereotypies [39]. The gene discussed is HDC; the disease is tic disorder.