Dolly, the world's first mammal cloned from an adult, has been euthanized, scientists said Friday.

A veterinary exam confirmed the six-year-old sheep had a progressive lung disease. Her cells had started to show signs of aging faster than a typical animal.

Researchers at the Roslin Institute in Edinburgh, Scotland, cloned Dolly, who was born in 1996. The birth was anounced on Feb. 23, 1997.

Sheep normally live to age 11 or 12, according to Dr. Harry Griffin of the Roslin Institute. Post-mortem tests are being done, and the findings will be reported, he said in a statement Friday. He added lung infections are common in older sheep.

Last year, Dolly's creators said the sheep had developed arthritis at a relatively early age, but was responding well to anti-inflammatory drugs.

Apart from the arthritis in the hip and knee of her left hind leg, Dolly had been a healthy animal that gave birth to six lambs, the team's creator, Dr. Ian Wilmut, said in Jan. 2002.

Scientists have found problems with cloned animals. Most attempts end in failure because the fetuses have oversized organs in the womb or are born stillborn.

Other cloned animals have been born twice as large as normal.

Last week, scientists in Australia announced their country's first cloned sheep had died unexpectedly. Post-mortem tests failed to identify a cause of death.

Cloning experts say it is important to know whether Dolly's disease was related to cloning technology.

The sheep deaths will likely add to the debate over whether using adult cells to clone animals inevitably causes errors in the copy.

In December 2002, the Roslin Institute warned of the dangers of cloning a human.

The Clonaid company announced the birth of the world's first human clone on Dec. 27, 2002. Scientists were skeptical of the claim, which hasn't been verified by independent tests.