Alterations in the brain’s histamine modulatory system have been implicated as a contributor to the development of tics and of TS by a series of genetic studies.6, 9, 10 The Hdc knockout mouse recapitulates a rare but high-penetrance genetic cause of TS and constitutes a promising model of its pathophysiology.6, 7Hdc-KO mice exhibit repetitive behavioral pathology, DA dysregulation, and altered markers of cellular activity and intracellular signaling in the striatum.7, 14, 43, 44. This evidence concerns the gene HDC and tic disorder.