The genetic relationship of attention-deficit hyperactivity disorder (ADHD) to Tourette's syndrome is less clear, with some evidence to suggest no genetic linkage, and some evidence to suggest that some forms of Tourette's syndrome may be genetically related to ADHD.[30] Not all persons with Tourette's syndrome will have ADHD or OCD, although in specialty clinics where the most impaired patients are treated, a high percentage of patients seeking treatment do have ADHD.[8] The high co-occurrence of ADHD observed in specialty clinics may be due to clinical ascertainment bias. Further study is needed to understand the genetic relationship between ADHD and Tourette's.[30]