Not just mice or sheep, but cow-humans too!
http://news.bbc.co.uk/1/hi/health/6121280.stm
I've been following chimera issues for a long time. I had a short blog post on above story last November.
http://grapesandfigs.blogspot.com/search?q=chimera
It used to be science fiction. There were oblique references to ethics. (Remember
"The Fly"? The human essence--or intelligence-- had been mixed into a fly's body which was under threat of being swatted without much thought.)
But in the meantime, while a few people wrung their hands and pondered, the practice has advanced now to a question of technique.