Scientists who successfully cloned Dolly announced the invention of a new method for replacing animal genes to create human organs for organ transplantation. Researchers at PPL Pharmaceuticals Inc. in Edinburgh, Scotland, said that they had cultivated four cloned sheep using a method of deactivating individual genes and replacing them with other genes. The researchers said that they also managed to close the genetic switch of pig cells, an innovation that makes it difficult for pig organs to be rejected by the body's immune system when used in human organ transplants. They also said that they have been able to put genes in specific locations on chromosomes. Dr. Neil Filst, a renowned expert in animal genetics at the University of Wisconsin, said that these two results have been successfully tested in mice. However, if you want to test success on larger animals, it is "very difficult." PPL Pharmaceuticals said that the next step would be to replace some specific genes with genes that the body uses.