SAN FRANCISCO Dec 23, 2005 — Scientists fretted Friday that a spectacular cloning fraud that hid in plain sight has set back legitimate stem cell work around the world. Cloning experts and stem cell scientists said research in the potentially revolutionary field of regenerative medicine will continue unabated. But they said public confidence in their work had been weakened by a sham branded by experts as the most visible case of scientific fraud they could recall. Scientists also struggled to explain how they didn't earlier catch the charismatic South Korean veterinarian's claim in a Science paper published in May that he cloned 11 human embryos to produce stem cells. "That's a difficult one," said Keith Campbell, the University of Nottingham researcher who helped clone Dolly the sheep in 1997. "Scientists are asked to referee a lot of papers and to a certain extent we have to believe each other as to the validity of the data." A disgraced Hwang Woo-suk resigned from Seoul National University on Friday after the school said the researcher fabricated groundbreaking cloning and stem-cell research that had raised hopes of new cures for hard-to-treat diseases. When Hwang published findings in February 2004 and again in May, it was believed he had developed a new cloning method that enabled him to accomplish something no one else had. Hwang chalked up much of the success to South Korean government support and dedicated researchers working around the clock. He also credited his workers' dexterity with chopsticks; stem cell researchers visited from around the world and rushed back to their labs to try the new technique. The journal Science said it would retract the May paper and investigate Hwang's 2004 paper that claimed the first cloned human embryo. "It's a stain on the honor and integrity of the whole field," said Dr. Robert Lanza, a cloning expert at the biotechnology company Advanced Cell Technology. "It has sent a lot of scientists on a wild goose chase and down false paths."