Still, he said, the fact that the technique involves adding genes to the cells adds some uncertainty.

Others are trying to add genes to healthy stem cells of cancer patients, to make them resistant to chemotherapy.

Adding genes usually made designer germs less potent, not more.

In a study of 12 patients, the researchers used inactivated cold viruses to add genes to cells that line the nose.

And scientists could only add genes, not take them away, restricting the method's uses.

Once they had the fetal cells they grew them in the laboratory and added new genes, at least one of which was human.

But they could not grow such cells in the laboratory and add genes to them.

There was no way that scientists could grow them in the laboratory and add genes to them.

But, he said, he is troubled by the idea of adding genes for certain complex traits.

Adding such genes might be akin to vaccinating a child.