Dolly, the sheep, the first animal cloned from an adult cell, has died aged only six, after doctors discovered she had a progressive lung disease typical of older animals.
Her death will refocus attention on whether cloned animals – and perhaps humans – die younger because their cells are in effect older than those that develop from normally fertilised embryos.
Dolly, who was put down by vets, created an upheaval in the world's view of genetics, birth and especially cloning. Now a number of groups around the world are claiming to have succeeded in cloning humans. Hundreds of animal clones exist, including cows, pigs, mice and goats.
Professor Rudolf Jaenisch, who in March 2001 co-wrote an article with Dolly's creator Professor Ian Wilmut for the journal Science titled "Don't Clone Humans!" said that the death "is exactly what was expected: clones will die early".
He added: "this is what we said two years ago. [Dolly] was sick. Clones have problems, and cloned humans would have problems ... People who claim you should clone humans should be stopped."
Dr Harry Griffin, head of the Roslin Institute near Edinburgh where Dolly was born and lived, said yesterday: "Sheep can live to 11 or 12 years of age and lung infections are common in older sheep, particularly those housed inside. A full post-mortem is being conducted and we will report any significant findings." Dolly was mostly kept indoors.
A key question will be about the effective age of Dolly's cells, which originated in those of a six-year-old ewe. Tests found in 1999 that Dolly's cells were already showing signs of wear more typical in an older animal.
Those were not the only indications of premature ageing. Last year she developed a lame leg due to arthritis, at the early age of five, although Professor Wilmut said last year that she remained healthy and had given birth to six lambs, which all appear to be healthy.
Dolly was born on 5 July 1996, but her existence was kept a secret while a formal scientific paper about the process that created her was prepared. The news finally leaked out on 23 February 1997, days before the paper was due to be published. She was named after the country singer Dolly Parton – because the original cell used to create her came from an udder.
Cloning remains difficult, and with unpredictable failures. Dolly was the only successful pregnancy of more than 277 embryos and 30 simultaneous embryo implants in different sheep. Deformed foetuses have died in the womb with oversized organs, while others were born dead. Still others died days after being born, some twice as large as they should have been.
Dolly's body has been promised to the National Museum of Scotland and will be put on display in Edinburgh.
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