We mutated that virus piece of DNA so that it was defective. It made defective proteins to which we made antibodies that allowed our ancestors to survive. Endogenous Retrovirus Mice on the other hand did not develop an immune response. There are strains of mice where female mice can spontaneously develop mammary carcinoma because of this retroviral gene. There are two groups of people, one in India and close to the Russian border who have virus particles in their lactating mammary glands.
Pigs have an endogenous retrovirus as well.
There are little carbohydrate and protein molecules sticking out of the surface of cell that you recognize, they are called histo-compatibility antigens. If you take some pig tissue and you pit it into human, you will develop an immune response and get a rejection of that tissue. If you take a recipient egg cell and remove the nucleus containing haploid DNA, you take a Donor Cell with a diploid nucleus, put it in the recipient egg cell. Dolly the sheep had 1000 or so nuclear transfers. They were planted into recipient female.