As Italian scientists succeed in creating the world's first horse clone, researchers in the UK have raised further questions about the future health of such animals.
Scientists in Cambridge have found new evidence that the process of creating an animal copy damages the genetic mechanisms that enable it to develop normally.
Apparently healthy, but for how long?
The discovery explains why it takes hundreds of attempts to create a living clone.
But it also has implications for the long-term health of these creatures. Many leading cloning scientists are becoming increasingly concerned that even apparently healthy animals may be flawed.
One scientist told the BBC that the death of Dolly the sheep last year was probably just an indication of what was to come: that many more cow and sheep clones would die as they approached middle age.
It is well known that most clones are abnormal and do not make it to term.
But the question has always been whether the supposedly healthy successful clones have more subtle defects that only show up later. The new Cambridge research shows this may well be the case.
The work involved producing a series of images illustrating how the signals in an embryo tell its cells to develop properly.
The Cambridge team found that the signals in clone embryos were abnormal. In most cases, these abnormalities prevent a clone from developing at all.
But Professor Wolf Reik, the head of the research team at the Babraham Institute, believes that some defects may only become apparent in later life.
"What could happen is that the clone is born looking quite normal and its early life is quite normal, but later on these animals could develop all sorts of diseases," he told the BBC.
Dolly suffered from premature arthritis
Dolly was created from an udder cell taken from an adult sheep.
The cell's genetic material was put inside an empty egg and given an electric shock.
In Dolly's case, this created an embryo which, once implanted into a ewe, led to a healthy lamb. But there were hundreds of failures on the way.
Most attempts did not produce viable embryos and there were some miscarriages and deformed births.
It was obvious even then that the cloning process damaged the genetic mechanisms of these failed clones.
There are now hundreds of animal clones, mostly cows such as those created by the US cloning firm ACT. They are soon going to reach Dolly's age.
Have they had any problems yet? ACT's scientific director, Dr Robert Lanza, says not.
But experiments by Professor Rudi Jaenisch of the Whitehead Institute for Biomedical Research in Cambridge, Massachusetts, US, show that mice at least have severe health problems as they get older.
Professor Reik believes his work backs Professor Jaenisch's.
Most of the animal clones in existence have been produced to develop medical treatments.
So many scientists argue that a little suffering on the part of the clones is worth the ultimate benefit to patients.
But with the prospect of horses being cloned purely for sport, Dolly the sheep's creator Professor Ian Wilmut believes the time is now right for a thorough and independent scientific programme to assess the harm posed to animals by cloning.