Cloning could make the genetic engineering of animals more accurate. Today, its success depends more on the hit-or-miss technique. Until recently, the method adopted for modifying most mammals was by injecting deoxyribonucleic acid (dna) into fertilised eggs. The dna would then integrate into the animal's genome at random, with unpredictable results.
Kenneth McCreath and his team at PPL Therapeutics near Edinburgh have described in Nature (July 1) how they first grew adult sheep cells in culture and made precise changes to targeted parts of the sheep genome. The team then cloned the altered cells by fusing them with egg cells from which the nuclei had been removed, and the cells grew into embryos.
Out of the 80 embryos implanted, three lambs survived for more than six months. "Gene targeting has only ever been done in mice before," says McCreath.
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