Extraordinary claims, scientists like to say, require extraordinary proof, and none has been more extraordinary in recent years than Scottish embryologist Ian Wilmut's claim that he and his colleagues had cloned a sheep named Dolly from a mammary cell of a pregnant ewe. More than a year later, nobody has managed to produce the Dolly experiment, and Wilmut is under growing pressure to prove that his famous sheep is what he says is she is. At a genetics meeting at the University of Louisville, in Kentucky, he blandly conceded that there was a "remote possibility" that there could have been a mix-up-that Dolly could turn out be the clone not of the adult ewe, but of the fetus the ewe was carrying.