It's the first time two women have shared in a single Nobel science prize. Over the years, a total of 10 women have won the prize in medicine.
Blackburn, 60, who holds U.S. and Australian citizenship, is a professor of biology and physiology at the University of California, San Francisco. Greider, 48, is a professor in the department of molecular biology and genetics at Johns Hopkins University School of Medicine in Baltimore.
London-born Szostak, 56, is a professor of genetics at Harvard Medical School and a researcher with the Howard Hughes Medical Institute.
Their work, done in the 1970s and 1980s, showed how features at the tips of chromosomes -- telomeres -- can keep them from getting progressively shorter as cells divide. It's been compared to the way plastic tips on the ends of shoelaces keep the laces from fraying.
Blackburn and Greider discovered an enzyme, telomerase, that maintains the lengths of the telomeres. Later research has shown that telomerase is switched on in almost all cancers.
Telomerase is active before birth, when cells are dividing rapidly. By age 4 or 5 it's basically shut off in almost all cells. That means the telomeres degrade over time, leading those cells to age and eventually stop dividing. But scientists have shown that adding telomerase to human cells can extend their lifespan indefinitely.
Such research spurred speculation that telomerase might turn out to be a fountain of youth. But experts say that aging is more complicated than just changes in telomeres. Scientists are still studying what impact telomeres might have; perhaps they will reveal ways to ward off some aspects of aging, researchers say.
Still other work showed that telomerase helps cancer cells sustain their uncontrolled growth. Scientists are trying to exploit that to produce new therapies, noted Jerry Shay of the University of Texas Southwestern Medical Center in Dallas.
The farthest along is a vaccine-like approach, which trains the immune system to home in on telomerase as a way to identify and attack cancerous cells. Other approaches attempt to use it as a signal that activates a cell-killing virus, or to devise a drug to block the enzyme's effect, he said.
Shay said he believes some kind of telomerase-based cancer treatment will become available within four years.
Monday's prize "is totally well-deserved," Shay said. "These people were clearly the forerunners of what is now becoming a much stronger field that has lots of interesting questions, (and that is) likely to have a major importance in medicine in the future."
The prize includes $1.4 million, split among the three winners.
Szostak, meeting with reporters, joked that he might use the money to send his two elementary school-age children to college. "They might like that," he quipped.
As for his work on telomeres, Szostak decided "it was time to move on" to another field. His current research is focused on the origins of life.
At a news conference in San Francisco, Blackburn joked that she had gone through the five stages of happiness after the phone rang in the middle of the night. "I went through, `Where's the phone?' to disbelief to dazed to, 'I think it's sinking in now," to, `I'm just so happy."'
Greider, in Baltimore, said she was telephoned just before 5 a.m. with the news that she had won.
"It's really very thrilling, it's something you can't expect," she told The Associated Press by telephone.
Later, she said the award was "really a tribute to curiosity-driven basic science."
Nobel judges say women are underrepresented in Nobel statistics because the award-winning research often dates back several decades to a time when science was dominated by men. Still, critics say the judges aren't looking hard enough for deserving women candidates.
The Nobel Prizes in physics, chemistry, literature and the Nobel Peace Prize will be announced later this week, while the economics award will be presented on Oct. 12.
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