Do you have a reference for the "injecting human DNA into animals" claim you make? It would be extremely disturbing, if true.
Researchers can take human cancerous (cells) and transplant them into an animal to study. So the animal has human cancerous cells growing on it.
The human DNA doesn't change the animal, it grows on top of it.
See: http://emice.nci.nih.gov/aam/mouse/transplantation-mouse-mod...