Yes, it’s a think now. Some mice are given human DNA through stem cells (often fetal) and some are actually grafted with human parts.
I did a lot of research on this when looking into monoclonal antibodies. If I am ever given a drug ending in -imab, I am going to do some deep research into how exactly it was developed. Here is a starting point:
https://en.wikipedia.org/wiki/List_of_therapeutic_monoclonal_antibodies
hmmm- I knew about grafting things like an ear onto mice- which i thought was weird, but didn’t realize they go as far as they do creating humanized mice-