TWO researchers in the Unites States who first discovered that genes could be split, opening the way for better treatment of cancer and other diseases, won this year's Nobel prize in Physiology or Medicine on Monday. American Phillip Sharp and British-born Richard Roberts share the 6.7 million Swedish crown (about HK$6.4 million) prize for independently discovering that a gene could be several units rather than one continuous segment. The prize is awarded by the Nobel Assembly of the Karolinska Institute to those whose research in medicine and physiology has most benefitted mankind. Last year, two Americans, Edmond Fischer and Erwin Krebs, were awarded the prize. They discovered a process of ''reversible protein phosphorylation'' that explains how an enzyme helps regulate the interaction of proteins in cells. It illuminated one of the basic regulatory mechanisms of human biology - a process involved when medicine prevents rejection of transplanted organs, or when certain cancers develop. The award, one of five endowed by the will of Swedish dynamite inventor Alfred Nobel, is commonly thought of as the ''medicine prize''. But it is also meant to honour discoveries in physiology - the study of how living bodies function - for which medicalapplications may still be years away. ''It's heart-rending to get letters from people who believe that drugs have arisen as a result of our work,'' said Mr Krebs after he and Mr Fischer won the 1992 prize. ''You have to understand the mechanism before you can find any cures to problems in the cells,'' said Mr Fischer, who claims both American and Swiss citizenship. In modern times, the prize most often has been shared, as decades of basic research and high-tech laboratories have begun to bear fruit. Of the 155 medicine or physiology prize winners from 1901 to 1992, 69 of them, 44.5 per cent, were Americans. Until 1933, the United States won no medicine prize. But since then Americans have dominated the awards, chiefly because of the nation's extensive research system, which also attracts top scientists from other countries. In 1987, the last time the award was given to a single recipient, winner Susumu Tonegawa of Japan was working at the Massachusetts Institute of Technology. The Nobel Prize in literature was won recently by novelist Toni Morrison, the first black American laureate. The Nobel Memorial Prize in Economic Science, established by the Central Bank of Sweden in 1968, were awarded yesterday. The other prizes established by Nobel in his 1896 will are the physics and chemistry prizes, to be awarded in Stockholm on Wednesday, and the Nobel Peace Prize, to be awarded in Oslo, Norway, on Friday.