Susceptibility to Tourette's syndrome is known to have a genetic influence. This study, of a nonconsanguineous family in which the father and his eight children are affected by the disorder, implicates a deficit in L-histidine decarboxylase activity as one potential cause of the disorder. This study of a nonconsanguineous family in which the father and his eight children are affected by Tourette's syndrome implicates a deficit in L-histidine decarboxylase activity as one potential cause of the disorder. Tourette's syndrome is characterized by childhood onset, waxing and waning symptomatology, and typically, improvement in adulthood. The molecular underpinnings of the disorder remain uncertain, although multiple lines of evidence suggest involvement of dopaminergic neurotransmission and abnormalities involving cortical–striatal–thalamic–cortical circuitry. 1 Current treatment focuses on tic reduction and management of prevalent coexisting conditions such as obsessive–compulsive disorder and attention deficit–hyperactivity disorder. However, therapeutic options have limited efficacy and may carry clinically significant side effects. Consequently, the development of new treatments based on an improved understanding of disease pathophysiology is a high priority. 2 The large genetic contribution to Tourette's syndrome is well established. . . .