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Wednesday marked the 20th anniversary of the announcement of Dolly the sheep, the first mammal cloned from an adult cell. Her creation left a lasting impact on both the public and the field of developmental biology, experts say. At the time, other researchers had managed to clone mammals by splitting embryos in a test tube and implanting them in adults. However, none had successfully used an adult somatic (body) cell to clone a mammal. Researchers at the Roslin Institute in Scotland were finally able to produce Dolly — cloned from the udder cell of an adult sheep — after 276 attempts, according to the National Human Genome Research Institute (NHGRI). "For a developmental biologist, the ability to clone an advanced mammal was thought to be impossible," Lawrence Brody, director of the Division of Genomics and Society at the NHGRI, told Live Science. Although Dolly was born in July 1996, Researchers announced Dolly's existence on Feb. 22, 1997. The delay in the announcement was due to the time needed to amass sufficient data on the project, check the data, write and get the manuscript published, said Bruce Whitelaw, the head of the Division of Developmental Biology at the Roslin Institute. Although British biologist John Gurdon had cloned frogs from the skin cells of adult frogs in 1958, researchers after him had failed to clone mammals such as mice, rats and pigs, despite trying for decades, Brody said. He added that many researchers began to feel there had to be "something different about mammals in the way their genome and genetic blueprints are packaged," and that cloning them would be impossible. However, Dolly's creation "told us that was all wrong," Brody said. That breakthrough would be crucial in the years to come. "It is rare that a single scientific story can have such a rapid, then sustained, impact" on science, Whitelaw said. Dolly had a massive scientific impact, especially through driving stem cell research and therapy, Whitelaw told Live Science.