The FDA says that's not allowed. Since 2015, Congress has forbidden the FDA from accepting submissions for clinical investigations that involve intentionally creating a human embryo with a heritable genetic modification. But Zhang's research does just this.

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The letter notes that Zhang — who is chief executive of a clinic called New Hope Fertility Center and a research company called Darwin Life — had already agreed not to create any more “three-parent” embryos in the United States, but the letter takes issue with the company's continued marketing of the service. On websites, the companies say the technology will “prevent maternally transmitted diseases” and help prolong “natural fertility.”

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“Other applications,” they note, “will be discussed in the future.”

While the technology was originally used to avoid a disease passed through mitochondria, organelles that are passed down through the egg cell, Zhang's clinic was hoping to use it for another purpose — to help women of advanced maternal age who had been unable to conceive in other ways. By using an egg from a younger donor combined with their own DNA, the women would theoretically be able to have their own genetic offspring.

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The controversial birth of the first “three-parent” baby was revealed in September 2016, and the science was described in more detail in an article in Reproductive Biomedicine Online in January 2017.

It's unclear why the FDA is weighing in now, almost a year after the experiment became public, although the Trump administration has taken a more conservative approach to reproductive health issues than previous administrations.

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The FDA and other government agencies have become more aggressive over the past few years in asserting their powers over emerging industries, but the extent to which their mandates, which were written decades before these technologies were imagined, give them control has been hotly debated. The FDA has been recently working with liquid biopsy companies that aim to detect early signs of cancer in blood and with personal genomic testing companies to ensure they are able to deliver on the marketing promises they are making to consumers.

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The United States remains among the most loosely regulated countries when it comes to sperm and egg donation and in vitro fertilization, and there aren't a lot of signs that will change anytime soon. The FDA letter deals with only one company and technology — mitochondrial replacement technology, or MRT — that can be used to form a genetically modified embryo, but things are moving so quickly in the reproductive sciences that experts say it's difficult to predict where we are headed.

Just last week, a team of researchers from Oregon Health & Science University and elsewhere reported that they had “corrected” the DNA of human embryos to erase a heritable heart condition that is known to cause sudden death in young competitive athletes. The FDA hasn't commented on that breakthrough.

This post has been updated.