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THE failure to properly reactivate and then deactivate one of the two X chromosomes could be why many cloned cows die soon after birth.

A team at the University of Connecticut in Storrs compared five cloned female calves that died shortly after birth with normal cows and healthy clones. They found the dead clones had genes on one X chromosome switched on that should be switched off and vice versa.

What鈥檚 more, in the placentas of the animals that died, both X chromosomes appeared active. This could be why the placentas of some cloned embryos are abnormally large, the team suggests.

Because female mammals have two copies of the X chromosome, one is normally completely shut down in each cell to prevent the overproduction of proteins coded for by the genes on the X chromosome. To clone a female animal, a cell from an adult with one X chromosome already inactivated is inserted into an egg that has had all its own chromosomes removed. The egg somehow switches on the inactivated X chromosome. Later, one of the X chromosomes is switched off again.

The latest research suggests that this reprogramming often goes wrong, at least in cows. The genes on the inactivated X chromosome may be only partially reactivated. If the other Xchromosome is then shut down the cell is left with only the damaged one.

Even with the latest research, it鈥檚 still not clear what鈥檚 going on, says William Rideout of the Whitehead Institute in Massachusetts. 鈥淲e don鈥檛 really understand reprogramming and what鈥檚 involved,鈥� he says.