Pigs are an attractive source for transplants because their organs are of a similar size to those of humans, but normally pig organs are rejected by the human immune system within an hour. The pig cells carry sugars on their surface that act as red rags to human immune cells. Genetic engineers can create pig cells without these sugars by disrupting the genes that encode the enzymes which process them. However, they haven't been able to turn those cells into whole animals. Nuclear transfer, the technique used to produce Dolly, was a promising approach, but making the technology work in pigs turned out to be problematic.
Sows require several viable embryos in a litter to carry a pregnancy to term.
And the electric pulse or chemicals that PPL used on other species to activate the egg with its implanted nucleus to begin forming an embryo did not work in pigs. Dave Ayares of PPL says that they had to modify every step of their normal nuclear transfer process-which cells to use as donors, how to grow them and how to unite egg and nucleus.