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A string of DNA.

Chinese scientist He Jiankui’s extraordinary claim two weeks ago that he had helped to make the first babies—twin girls—with edited genomes shocked the world. Many questions remain about the experiments, but among researchers’ chief concerns are the potential effects of the genetic alterations on the girls’ health.

He, a genome-editing researcher at the Southern University of Science and Technology of China in Shenzhen, says in several YouTube videosthat he impregnated a woman with embryos that had been edited to disable a gene that allows HIV to infect cells. He targeted this gene, known as CCR5, because it is well studied, and because its mutation offers protection against HIV infection, which still carries a significant social stigma in China.

The CCR5 gene has been the subject of research since the mid-1990s, and has roles beyond HIV that scientists are just beginning to understand. Loss of CCR5 function increases the risk of severe or fatal reactions to some infectious diseases, for example, and has also been shown to enhance learning in mice.


The CCR5 protein is expressed...