A team of researchers based at the University of Hawaii has shown that the ageing process is not accelerated in successive generations of cloned mice. Earlier studies carried out on Dolly the sheep indicated that cloned animals might age prematurely. But the US scientists, who published their results in last week's Nature, found that six generations of sequentially cloned mice aged normally.

The mice were cloned using SCNT (somatic cell nuclear transfer), the same technique used to create Dolly. Each time a cell divides to make two new cells, the ends of its chromosomes (telomeres) become shorter, until they become so short that the cell dies. But while Dolly's telomeres are shorter than expected for her age, those of the cloned mice are slightly longer than normal. Teruhiko Wakayama, who led the team, is not sure why the telomeres are longer, or if the mice will have a longer lifespan than normal.

The fifth generation mouse lived to be middle-aged, but the sixth generation clone was eaten by its foster mother, bringing the experiment to an abrupt end. 'It is a preliminary but very important study that needs to be repeated' said Tony Perry, another member of the team.