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Baby having its heartbeat checked by a doctor

There are new concerns about the world's first genetically modified babies.

It appears that the genetic variation a Chinese scientist was trying to recreate when he edited twin girls' DNA may be more harmful than helpful to health overall, according to a study published Monday. The study, in Nature Medicine, involves the DNA of more than 400,000 people.

"This is a cautionary tale," says Rasmus Nielsen, a professor of integrative biology at the University of California, Berkeley, who led the new study.

The Chinese scientist, He Jiankui, announced last fall that he had created twin girls from an embryo whose DNA he edited in his lab using the powerful gene-editing tool CRISPR.

He said he modified a gene known as CCR5 to protect the girls from the AIDS virus. But there's also evidence the CCR5 variation has other effects, such as making people more vulnerable to the West Nile and influenza viruses.

"We know it has many different effects. The question is: Is it overall beneficial or detrimental to have this mutation?" Nielsen says. "That was not... see more