Clones may be defective, scientists warn

Genetics experts say clones that look normal may hide serious defects which make the science unsafe.

They found that some clone cells developed poorly because the embryo stem cells used in the process were unstable.

As they divided, they were found to lose the ability to tell secondary 'imprinted genes' whether they should be turned on or off.

Although these genes don't affect the base sequence, and therefore the basic development of the clone, they may cause unseen problems.

Until now it has always been thought that embryo stem cells would be 'purer' than adult cells and would therefore clone better.

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.

The researchers said the findings raised major doubts about reproductive cloning, but were unlikely to influence therapeutic cloning.

Rudolf Jaenisch and colleagues carried out the work on mice at Whitehead Institute for Biomedical Research and the University of Hawaii.