I had some mice recently losing hair on the back hind, and it was maybe 4-6 of them. I was kind of worried, and put them down in case they had something contagious. That was 3 days ago. Today, I saw one of my baby rats with the same hair style on its head. I'm not sure if it was just wet, but I'll monitor it tonight. I still am unsure. I doubt it's the rex genes though.