Cloning mammals was once thought impossible because, as cells in the body develop and specialise, various chemical tags that control gene activity are added to, or removed from, parts of the genome. Skin cell DNA, say, is “programmed” to make skin cells. But the birth of Dolly the sheep in July 1996 showed that transferring the nucleus of an adult cell into an empty egg could reprogram its genome and allow the egg to develop. Soon afterwards, Wakayama created Cumulina, the first cloned mouse, born in October 1997.