In the latest report concerning the health of cloned animals, Japanese scientists have found that cloned mice seem to have a shortened life span as a result of compromised immunity.

Of 12 mice that were cloned, 10 died before reaching 800 days. In comparison, only three out of 13 control mice died in the same period.

Post-mortem examination of the cloned mice showed that six had pneumonia, four had serious liver damage, one had leukemia, and one had lung cancer.

Writing in an advance online publication of the journal Nature Genetics, a team led by Atsuo Ogura of the National Institute of Infectious Diseases in Tokyo suggested that the deaths may have stemmed mainly from a deficiency in the immune system.

Other recent reports have shown that even though cloned animals may appear normal, they can develop anomalies later in life. For example, adult obesity and elongation of certain parts of chromosomes, called telomeres, have been observed in cloned animals.

The consequences of cloning can be "subtle, specific, and delayed," wrote Tony Perry of Advanced Cell Technology in Worchester, Massachusetts, in a commentary accompanying the Japanese study.

Dolly the sheep—the "poster animal" of cloning—appeared normal at birth and even gave birth to six healthy lambs. But she now has arthritis, a disease that usually occurs in sheep much older. Scientists don't know whether the disease is a result of the cloning or a random occurrence.

Referring to the results of the new Japanese study, Rudolf Jaenisch of the Whitehead Institute for Biomedical Research in Cambridge, Massachusetts, said: "I'm not surprised at all. Clones have many defects even if they look normal."

Yet cloned animals don't necessarily have problems such as a shorter life span or heightened risk of disease.

"Two of the cloned mice are still alive and may have normal life spans," said Perry. "These cloning-associated anomalies need to be studied and understood—many problems may eventually be circumvented."

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