By gene editing mouse sperm cells first, and then using corrected sperm to make embryos, Li’s mice came out perfect every time.

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Yet some scientists agree that sperm may be the most practical avenue for making designer people. “It could turn out to be a very good option,” says George Church, a professor at Harvard University and one of the inventors of the CRISPR technology.

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“There is no reason to think it can’t be done,” says George Daley, a noted stem cell researcher at Harvard University’s medical school. From what he’s seen, he says, the prospect of installing custom DNA edits into lab-grown reproductive cells is “very real.”

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