I got my hands on a small group of what I believed to be "fuzzy hairless" mice. These are a little more difficult to get than the regular hairless, so I was really stoked about it. Imagine my disappointment when they slowly started getting their hair in. >.< I'd say give them a couple of weeks. If they haven't grown their hair in by then, it is likely you could have some of the hairless or even "fuzzy hairless". If they grow hair...then you were just unlucky enough (like me) to have just gotten slower growing mice.