3. It’s not clear what those new mutations will do.

At least two of the three mutations that He introduced into Nana and Lulu’s genomes are substantial changes that could alter how CCR5 works. Typically, scientists would introduce the same mutations into mice or other lab animals to see what would happen. If they felt reassured enough to move into human patients, they could recruit patients with HIV, take out some immune cells, introduce the new CCR5 mutations, transplant the cells back, and monitor the volunteers to see if they’re healthy. “That could take months or years, but to do anything less would be cutting corners,” Ryder says.

But He appears to have leapfrogged over all of those basic checks and implanted the edited embryos into a woman. “The children are test subjects for variants that haven’t been vetted in animals,” Ryder says. What’s shocking about this “is the blatant disregard of all the rules and conventions we have in place for how one should approach any proposed intervention,” said Leonid Kruglyak, a geneticist at the University of California at Los Angeles, on Twitter.