Paris - Four genetically-identical copies of Dolly the famous cloned sheep, which suffered ill health and died prematurely in 2003, are going strong at the advanced age of nine, a study said Tuesday.Debbie, Denise, Dianna and Daisy - identical sisters of Dolly, though born 11 years later - were "in pretty good health", according to researchers who studied whether cloned animals can live long, healthy lives.The quadruplets were made from the same mammary gland cell line that yielded Dolly - the first mammal cloned using a technique called somatic-cell nuclear transfer (SCNT).Born 20 years ago on July 5, 1996, Dolly developed crippling knee arthritis aged five and died of lung disease at the age of six - about half the life expectancy of her breed of Finn-Dorset sheep.Dolly's ill health and early demise raised red flags that clones may be sickly and age prematurely compared to naturally-conceived peers.Cloned lab mice, too, have shown a propensity for obesity, diabetes, and dying young.Kevin Sinclair of the University of Nottingham and a team conducted thorough medical exams on the four "Dollies" born in July 2007, as well as nine other sheep clones from different cell cultures.All 13 animals, aged seven to nine, were the product of lab studies seeking to improve the efficiency of SCNT.Experts measured the sheep's glucose tolerance, insulin sensitivity, blood pressure and muscle and bone strength, in what they said was the first comprehensive assessment of age-related disease in animal clones.A few of the sheep had mild osteoarthritis, the team found, and one a "moderate" form of the ailment -- though this was not unusual for their age. None of the sheep were lame, as Dolly was.Ageing 'normally' Despite their "advanced age", none of the sheep were diabetic, and they had normal blood pressure. The data, said the team, was "compelling, indicating no detrimental long-term adverse effects of SCNT on the health of aged adult offspring".SCNT involves removing the DNA-containing nucleus from a cell other than an egg or sperm - a skin cell, for example - and implanting it into an unfertilised egg from which the nucleus had been removed.Once transferred, the egg reprogrammes the mature DNA back to an embryonic state with the aid of an electric jolt, and it starts dividing to form an embryo.No human is known to ever have been created in this way.In spite of recent improvements, the technique remains inefficient, and expensive, with a small percentage of cloned embryos surviving to birth."For those clones that survive... however, the emerging consensus, supported by the current data, is that they are healthy and seem to age normally," said the study in the journal Nature Communications.Animal cloning is used in agriculture, mainly to create breeding stock, as well as in the business of "bringing back" people's dead pets. Despite initial high expectations, it has not found a place in the field of medical therapy for humans.