Get stuffed: Dolly will stay in the public eye. Credit: PA

The fate of Dolly the sheep is now certain: with the post mortem complete, her body will be stuffed and put on display at the Museum of Scotland in Edinburgh, UK. But her death has drawn attention to how little is known about the health problems that other clones may or may not suffer.

Dolly was put down on 14 February, suffering from a virus that caused a tumour in her lung. Ian Wilmut, leader of the team that created Dolly at the Roslin Institute near Edinburgh, says that the post mortem revealed no other gross abnormality, apart from her arthritis, which was diagnosed last year.

Dolly, born six years ago, was the first mammal to be cloned, but since then six other mammalian species — cow, mouse, pig, rabbit, goat and domestic cat — have also been copied. The cloning process is inefficient: only around 3% of cell nuclei that are transferred to donor eggs result in live births. But no large-scale follow-ups of those births have been done, so little is known about whether clones really are likely to die young.

The health of adult clones has been studied most extensively in mice. At least 200 live mouse clones have been created in 10 or so labs around the world. In one study, Narumi Ogonuki of the National Institute of Infectious Diseases in Tokyo tracked the health of 12 mouse clones and found that 10 died — probably of pneumonia or liver disease — before their average natural lifespan of 800 days (N. Ogonuki et al. Nature Genet. 30, 253–254; 2002). But other researchers say that problems seen at birth or shortly afterwards, such as obesity, can reverse as the animals age. The offspring of other cloned mice have also been reported to be normal (K. L. K. Tamashiro et al. Nature Med. 8, 262–267; 2002).

Researchers say that experiments with larger numbers of animals are needed, but are difficult to carry out given the difficulties associated with creating clones. Work on some animals is hampered by a lack of time and money. “You need to follow them for life. We're talking 15-plus years for cows,” says Keith Campbell, a cloning researcher at the University of Nottingham, UK.