Eighteen years ago, scientists in Scotland took the DNA from an adult sheep and put it into the egg of another sheep. That egg with a new DNA was then implanted in the womb of a third sheep, and the result was Dolly, the first clone of a mammal.

Every since, scientists have been trying to do the same thing with humans, not to produce people but to produce cells useful for medicine. NPR's Richard Harris reports on the latest step in that direction.

RICHARD HARRIS, BYLINE: Dolly's birth set off a huge outpouring of both ethical concern and medical hope and also frustration. It's proven very difficult to do that same sort of DNA transfer into a human egg. Last year, scientists in Oregon said they'd finally done it, using DNA taken from infants. Robert Lanza, chief scientific officer at Advanced Cell Technology, says that was an important step but not ideal for medical purposes.

ROBERT LANZA: So there are many diseases, whether it's diabetes, Alzheimer's or Parkinson's disease, that usually increase with age. So we're very likely to need to be able to use these techniques from an adult.

HARRIS: Lanza's colleagues in Korea and Los Angeles now report they've done just that. Writing in the journal Cell, Stem Cell, they say they started with DNA extracted from the skin of two men and injected it into human eggs donated by four women.

LANZA: What we show for the first time that you can actually take skin cells from a middle-aged, 35-year-old male, but also from an elderly, 75-year-old male.

HARRIS: And used the DNA from those cells in this cloning process. They injected it into 77 human egg cells and managed to create two cells with the men's DNA, cells that could keep dividing indefinitely.

LANZA: So from a small vial of these cells, we could grow up as many cells as we would ever want.

HARRIS: They look like the cells in a human embryo. And with a bit of coaxing, these cells could be turned into nerve cells, pancreas cells, liver cells, heart cells, just about anything. And that's what makes them potentially useful for treating all sorts of diseases.

Now, in the 18 years since Dolly, scientists have found another way to produce these human cell lines, as they're called. And the other technique skips the step involving human egg cells, so some people find it less fraught, ethically. It also means that finally getting the sheep technology to work with adult human cells may not be a big turning point.

LANZA: We now have two ways of doing this, and we're not sure which of the two methods is likely to work best.

HARRIS: Ideally, Lanza would like to screen millions of adults and choose just a hundred or so whose genes would make them good DNA donors. He'd like to see a library of cells with those carefully chosen genes to reduce the risk that tissue would be rejected if it's transplanted into an unrelated person.

Paul Knoepfler, at the UC Davis School of Medicine, is excited about this advance from a medical point of view. But he says it does mean we could be getting closer to being able to clone an entire human being.

PAUL KNOEPFLER: I don't think that's coming anytime soon but certainly this kind of technology could be abused by some kind of rogue scientist.

HARRIS: And while many people consider that idea repugnant, it is not broadly illegal. Richard Harris, NPR News.

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