Americans Elizabeth H. Blackburn Carol W. Greider and Jack W. Szostak jointly won the 2009 Nobel Prize in medicine on Monday for discovering a key mechanism in the genetic operations of cells an insight that has inspired new lines of research into cancer. It was the first time two women have been among the winners of the medicine prize in the same year. The trio working in the late 1970s and 1980s solved the mystery of how chromosomes the rod-like structures that carry DNA protect themselves from degrading when cells divide. The Nobel citation said the laureates found the solution in the ends of the chromosomes - structures called telomeres that are often compared to the plastic tips at the end of shoe laces that keep those laces from unravelling. Blackburn and Greider discovered the enzyme that builds telomeres - telomerase - and the mechanism by which it adds DNA to the tips of chromosomes to replace genetic material that has eroded away. The prize-winners work set the stage for research suggesting that cancer cells use telomerase to sustain their uncontrolled growth. Scientists are studying whether drugs that block the enzyme can fight the disease. In addition scientists believe that the DNA erosion the enzyme repairs might play a role in some illnesses. Blackburn who holds US and Australian citizenship is a professor of biology and physiology at the University of California San Francisco. Greider is a professor in the department of molecular biology and genetics at Johns Hopkins University School of Medicine in Baltimore. London-born Szostak has been at Harvard Medical School since 1979 and is currently professor of genetics at Massachusetts General Hospital in Boston. He is also affiliated with the Howard Hughes Medical Institute the citation said. Ten women have won the prestigious medicine award since the first Nobel Prizes were handed out in 1901 but this was the first time that two women were honoured in the same year. The award includes a 10 m million kronor 1.4 million US dollars purse divided among the winners a diploma and an invitation to the prize ceremonies in Stockholm on December 10. 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 October 12.