BOB ABERNETHY: The first cloned pigs were announced this week in Virginia at a laboratory owned by the same company that cloned Dolly the sheep four years ago. The five piglets were all grown from a single adult cell. In theory, if big transplanting problems can be overcome, organs from cloned pigs -- hearts, livers, kidneys -- could be used someday to replace damaged parts in human beings.

How real is that possibility, and what are the problems, scientific and ethical? Rick Weiss covers biomedical issues for THE WASHINGTON POST.

Rick, welcome. Why does cloning these pigs lead us closer to being able to using pig organs as human transplants?

Mr. RICK WEISS (THE WASHINGTON POST): You know, the biggest scientific hurdle to using these animals as organ donors is that their organs are rejected very quickly by the human immune system. They're just too different. So, in theory, one can design a pig, engineer a pig to be more compatible by adding some genes and taking some genes out of the pig. But it's a lot of work. And once you pull that off, it'd be a lot nicer to be able to clone that transplant donor pig and make thousands of duplicates than to go through all that work over and over for every animal donor that you need.

ABERNETHY: So what we know is if they can solve the rejection problem, then they can do the ...

Mr. WEISS: Right.

ABERNETHY: ... the cloning. People worry that parts from an animal might carry with them disease, perhaps very serious diseases.

Mr. WEISS: There are some issues there. Pigs do carry certain kinds of viruses that do not typically live in people. And there are concerns that those viruses, if transplanted into people, might cause disease, or even larger concerns that those viruses might genetically intermingle with human viruses inside a recipient, create new offspring viruses that have never existed before, and perhaps infect people in the general public. It raises an ethical question of whether the public should somehow have a say in whether this person is going to get a transplant.