Enlarge Image Too clean. Both mice lack the gene Hoxb8, but the animal on the right has received bone marrow from a healthy mouse, curbing its tendency to groom compulsively. Credit: Shau-Kwaun Chen/University of Utah School of Medicine Some people just can't help themselves. They wash their hands over and over, scrubbing their skin raw. Or they lock and relock doors, pull out their own hair, or obsessively rearrange the contents of their closet. Now, a study of mice suggests that faulty immune cells prompt such compulsive behaviors. The results raise the possibility of treating obsessive-compulsive disorder by targeting...