Progress in the elucidation of the genetic mechanisms involved in the expression of obsessive-compulsive disorder (OCD) has been hampered by 2 factors: heterogeneity of the clinical phenotype and lack of understanding of the molecular mechanism of the disorder. Efforts to narrow the phenotype have included identification of 4 symptom dimensions as well as investigation into comorbid and perhaps related illnesses such as Gilles de la Tourette's syndrome, eating disorders, and impulse-control disorders. A number of familial studies have been conducted to explore the relationship of OCD to these illnesses and the possible existence of independently heritable components that make up the more complex disorder. Candidate gene studies are also being conducted, with the goal of understanding the molecular mechanisms of OCD.