Female cloned cows that die just after birth have unusual gene expressions, whereas those survive the cloning process have normal patterns, a new study has found.

The research is led by Xiangzhong Yang, a cloning expert who was born in China, at the University of Connecticut in the United States. The finding is published Sunday online in the journal Nature Genetics.

The researchers studied the patterns of gene expressions on the X-chromosome in female cloned cows.

Female animals usually carry two X-chromosome, each from one oftheir parents. In natural reproduction, the genes on the two X-chromosome are active in female embryos. One X-chromosome will then be silenced to ensure that cells in females have the same amount of gene expressions as cells in males, who carry a single X-chromosome and one Y-chromosome.

In female cloned animals created using the genetic material of an adult, one X-chromosome is already silenced or inactivated, andmust be completely reprogrammed and then inactivated again later in development.

Previous studies have indicated that this process of X-chromosome reprogram and inactivation is normal in cloned mouse embryos.

But Yang and his colleagues found that in cloned female cows that die shortly after birth, 9 of the 10 genes they looked at on the X-chromosome have abnormal inactivation patterns.

When they looked at expression of the same genes in clones thatlived and in control animals that had been conceived through natural reproduction, they saw no differences.

"The results may explain why cloned animals often die soon after birth," the researchers said.

(People's Daily May 27, 2002)