Tourette Syndrome (TS) is a childhood-onset neurodevelopmental disorder affecting between 1:300 and 1:400 individuals1–3. TS has a complex etiology, with both genetic and environmental factors contributing to its expression4–12. Although multiple motor and vocal tics form the core of the diagnosis, individuals with TS often present with a variety of additional symptoms and comorbid disorders, most commonly obsessive-compulsive disorder (OCD), subclinical obsessive-compulsive behaviors or symptoms (OCB), and attention-deficit hyperactivity disorder (ADHD)13. Despite genetic epidemiological studies showing strong evidence of familiality, definitive susceptibility genes for TS have not yet been identified. One question that has not yet been satisfactorily answered is whether TS, OCD, and ADHD share susceptibility genes. OCD and/or ADHD are present in up to 50% of individuals with TS, and are enriched in TS families14–17. While segregation analyses and family studies provide strong support for a genetic relationship between TS and OCD, the data are less clear with regard to the relationships between TS and ADHD and between OCD and ADHD4–6, 12, 18–31. For example, family studies have shown that ADHD alone is not increased in TS families;
