I recently came across a story about a controversial new science project: growing human organs inside pigs, cows, and sheep. It works by first removing the gene that a pig needs to make the desired organ–say, a pancreas. Once the scientist has a pig embryo lacking that gene, he injects human stem cells into the embryo, hoping that the human stem cells will fill in the gaps and make a human pancreas. The ultimate goal is to transplant those organs into humans.
The project is so controversial that the National Institutes of Health has refused to fund it. The researchers are