What if science could create what science fiction writers could only imagine? A chimera: a creature that's part beast, part man.
Well, as CBS News Correspondent Byron Pitts reports, here it is.
"They have human cells in their liver and pancreas … (and) in their heart," says professor Esmail Zanjani. "As you see, they're perfectly normal sheep."
At the University of Nevada, Zanjani and his colleagues are implanting human stem cells into sheep before birth. The cells then grow into partial human organs.
In a way, he says, it's tricking Mother Nature.
The goal here is not to create talking sheep, but rather scientists see this as an opportunity to grow human tissue in organs outside the human body, where the medical benefits could be endless.
"We are hoping that we could be able to generate enough insulin-producing cells that could be sufficient to transplant (to) a patient with diabetes," says Zanjani.
Inside the animal pen, Zanjani says, could lie "some possibilities of treatment" for human illnesses.
But critics of this kind of science have their own questions, not simply "What if" but "What next?"
"If we open this door and say it's alright to create new forms of life that are part human and part animal, what will we ever say no to?" says economist Jeremy Rifkin, author of "The Biotech Century."
Rifkin is an anti-biotech activist. He calls such experimentation "scientific blackmail."
"The scientists here always say if we don't use this radical experiment there's no other way to cure the disease," says Rifkin. "It's time to say to the scientist and the corporations doing this research, 'You know you don't have a blank check to do whatever you want to the human race.'"
And there lie the questions bioethicists, like Jason Robert of Arizona State University, struggle with everyday. Where's the line? When does science go too far?
"Well it's not clear. The line has to be somewhere in between a mad scientist running around trying to create new species and a complete and utter ban on all such research."
So some would say let sheep be sheep, and man be man.
"Yes, I understand that, but some of these animals have sustained us throughout life as food and now they're serving an even more noble cause perhaps," says Zanjani.
It is a brave new world. The great minds of science may have opened the door, but it may be up to the courts and members of Congress to guard it.