This is really interesting. It points to something not working correctly that had not been suspected in OCD before. And it was apparently serendipitous as well.

Gene Triggers Obsessive Compulsive Disorder-Like Syndrome in Mice: Study Suggests New Treatment Targets: National Institute of Neurological Disorders and Stroke (NINDS)
Using genetic engineering, researchers have created an obsessive-compulsive disorder (OCD) - like set of behaviors in mice and reversed them with antidepressants and genetic targeting of a key brain circuit. The study, by National Institutes of Health (NIH) -funded researchers, suggests new strategies for treating the disorder.