If mice are given human DNA, does it become a problem to kill them?

You’ve probably seen the picture of the mouse with a human ear growing on its back. When we tinker with the mouse DNA so that it is part human, with a human blood type, and a human liver growing inside it, or even a human brain eventually… at what point should people feel a moral quandary when killing them? It’s not just hypothetical.
http://news.bbc.co.uk/2/hi/health/1949073.stm