[Below is the original script. But a few changes may have been made during the recording of this audio podcast.]

Since Dolly the sheep was cloned back in July of 1996, the world of manipulating animal DNA has come a long way. In Massachusetts, goats now produce milk with drugs embedded. There are monkeys whose DNA glows green to enable scientific study.

But whether scientists are copying animal DNA in its entirety or simply manipulating parts of it, working with the genetic code of animals has proven trickier than initially thought.

Thus far, no genetically modified animals have thrived in the wild—unlike some of the GMO plants that have spread beyond test plots. Dolly still remains one of the only truly successful clones.

But transgenic animals, from mice to pigs, have proven vital to medical research. The U.S. Food and Drug Administration aims to ensure the safe use of transgenic animals and approved the first drug from transgenic goat's milk—an anti-clotting protein—earlier this year.

It's important to remember that we've been doing this kind of genetic manipulation of animals for a long time. Think: breeding. But it remains to be seen whether transgenic animals will provide miracle drugs, better food or even less polluting pigs. And even if we can make this work, there's still the potential problem of whether animals with altered DNA can be safely controlled.

—David Biello