You have a junk drawer full of stuff you might use some day. You have a few things that you know you'll never use again, but can't bring yourself to throw away. You have a collection that's too much and too big for the space you live in. So what makes you different from a hoarder? Not much.
Mice are also susceptible to obsessive-compulsive disorder, resulting in constant grooming that leaves them hairless and sore. Now some obsessive-compulsive mice were treated by replacing their bone marrow, demonstrating the first cause-and-effect link between the immune system and mental illness.