A new study has found that genes used in cloning have a high rate of failure, raising questions about scientists' ability to safely use cloning to treat human disorders.

The study in the journal Science said genes used in cloning often caused serious abnormalities in mice and even the use of embryonic stem cells did not assure the creation of normal mice. The report comes as the Bush administration considers whether to allow federal funds for non-cloning embryonic stem cell research.

David Humpherys, first author of the study, said that many of the mice cloned in the experiment appeared to be normal, including having normal genes, but there was evidence that during embryonic and fetal development the genes did not work properly.

Humpherys said there was no evidence that the genes in the cloned animals were altered, but that the way in which the genes made proteins was flawed and unstable. In effect, the researchers found that even though the biological blueprint was intact in the cloned animals, the way that the blueprint was read and interpreted was flawed. This could result in abnormal tissues and organs, they said.

"It is quite likely that just the animals that are most nearly normal make it to birth (in cloning), but our study shows that doesn't mean they are completely normal," said Humpherys. "There may be changes in gene expression that could affect them later in life."

In cloned humans, senior author Rudolf Jaenisch said the gene expression flaws could affect personality, intelligence and other human attributes.

"This study confirms the suspicions of many of us that cloning of humans would be really dangerous," said Jaenisch, a researcher at the Whitehead Institute for Biomedical Research and at the Massachusetts Institute of Technology.

Humpherys and Jaenisch said that a number of scientists doing cloning experiments with mice, pigs, sheep and cattle have reported that even apparently normal animals develop disorders later in life. Jaenisch said that extreme obesity has developed in many cloned animals, including Dolly, the first mammal cloned from an adult cell.