Shortly after a statement in the US that the first cloned human would be produced "very soon", scientists have issued a new warning about the effect of creating cloned babies.
Researchers found evidence that even healthy, normal looking animal clones are likely to have hidden genetic flaws and therefore the cloning of duplicate individuals may be intrinsically unsafe, the findings suggest.
The scientists were investigating why so many cloned animals failed to survive or grew to a grossly distorted size. They found that cloning affected the way certain important developmental genes were switched on or off by chemical "tags".
The workings of these genes varied significantly in the placentas and kidney, heart and liver of cloned mice, compared with normal mice and those born as a result of in-vitro fertilisation.
The genetic instability also showed up in the embryonic stem cell lines used to create the clones, the researchers reported in the journal Science.
Stem cells, precursor cells with the ability to become different kinds of tissue, are currently being investigated for possible therapeutic uses.
"Therapeutic cloning" of stem cells aimed at developing replacement tissue treatments has been given the go-ahead in Britain, while reproductive cloning is banned. Creating clones such as Dolly the Sheep involves removing the nucleus of an egg and replacing it with the nucleus from an adult cell, or embryonic stem cell.
The US team found that even clones made from sister stem cells had differences in their gene expression. Yet many embryos survived to adulthood, suggesting that developing mammals were surprisingly tolerant to faulty gene regulation.
"This suggests that even apparently normal clones may have subtle aberrations of gene expression that are not easily detected in the cloned animal," said Mr Rudolf Jaenisch from the Massachusetts Institute of Technology in Cambridge, USA.
In an interview with the USA today newspaper, Ms Brigitte Boisselier, a director of the Clonaid company which is engaged in a human clonding project, said that the first cloned human would be produced "very soon".