This invention provides in vivo and in vitro methods of screening for an agent or a combination of agents that reduces one or more repetitive behaviors, comprising contacting neuronal cells of an animal with a HOXB8 gene mutation with the agent combination of agents to be screened, and determining whether one or more repetitive behaviors of the animal is reduced or whether one or more biochemical correlates of repetitive behaviors is reduced, the reduction in one or more repetitive behaviors or biochemical correlates indicating an agent or combination of agents that reduces repetitive behaviors. The invention also provides method of treating a subject with repetitive behaviors, comprising administering a therapeutically effective dose of the agent or combination of agents identified by the screening method. Further provided is a population of animals with a HOXB8 gene mutation, wherein more than 30% of the animals have excessive grooming behaviors and wherein less than 10% of the animals show skeletal defects.