Dolly the sheep, the first mammal cloned from an adult cell, died on 14 February. Her caretakers at the Roslin Institute in Scotland euthanized the 6-year-old sheep after diagnosing an incurable lung tumor. Her death seems unconnected to the fact that she was a clone; the tumor is caused by a virus that had infected both cloned and normal sheep at the institute.
Dolly, born in July 1996, was created when Ian Wilmut, Keith Campbell, and their colleagues inserted the nucleus of a mammary cell taken from a 6-year-old ewe into an egg cell from which the nucleus had been removed. Wilmut says that a preliminary postmortem investigation of Dolly turned up only two abnormalities: the lung tumor and signs of the arthritis she had developed several years ago. Although a detailed evaluation of all her tissues is under way, Wilmut says, it will be difficult to ever determine whether the arthritis was affected by her origins as a clone.
Earlier reports from Wilmut and his colleagues that Dolly might be aging faster than normal might have been mistaken, he now says. In 1999, the group reported that Dolly's telomeres, the DNA caps on the end of chromosomes, seemed shorter than normal--a possible sign that the cloning process had affected the life expectancy of her cells. "We may have inadvertently been misled" by the difference in telomere length in different tissues, Wilmut says. Other researchers have shown that telomeres of cloned animals are either normal or longer than their noncloned counterparts (ScienceNOW, 27 April 2000).
Dolly is survived by six healthy offspring fertilized the conventional way. As agreed several years ago, the body will be stuffed and put on display at a science museum in Edinburgh. Wilmut and his colleagues are continuing to push the frontiers of nuclear transfer: They are applying for one of the first licenses in Britain to begin nuclear transfer experiments with human cells. For now, however, they are concentrating on smaller animals. Roslin suspended its livestock cloning program 18 months ago, Wilmut says, and is concentrating on detailed analysis of the nuclear transfer process in mice.