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Have we pushed the boundaries too far in innovative baby-making? Think back to when critics charged that the inventors of in-vitro fertilization recklessly “played God” by daring to combine a sperm and an egg in a lab to create Louise Brown in 1978. Now some 5 million of the world’s babies have been conceived via IVF. But it’s one thing to get used to combining reproductive parts in a lab; it’s a lot less comfortable to imagine tinkering with those parts beforehand. In an open letter to the U.K. Parliament, Paul Knoepfler, stem cell and developmental biology researcher at the University of California Davis School of Medicine, warned that supporters “could well find themselves on the wrong side of history … with horrible consequences.”

Yet it’s important to understand that mitochondrial replacement isn’t genetic engineering run amok, cautions Debra Mathews of the Berman Institute of Bioethics at Johns Hopkins University. The mitochondrial energy-making material of an egg accounts for a mere 37 genes, compared to the nucleus, which contains about 23,000 genes. “No one is messing directly with genes,” she says. “Scientists are replacing damaged mitochondria with healthy mitochondria. It’s a specific technology for a specific application. We’re modifying eggs to avoid serious diseases.” So far, researchers haven’t attempted a pregnancy using the technique, but a study published in 2012 inNature found that resulting embryos appeared to develop normally with the nucleus intact and did not contain any of the mutated mitochondria from patients’ previous eggs. And scientists at Oregon Health and Science University transferred the mitochondria between rhesus-monkey eggs and created four healthy monkey babies.

Yet determining when a technology is “safe” is especially challenging in fertility medicine because the only way to find out is to create another human. The FDA’s prudence is a welcome change from the early “wild west” days of reproductive medicine when many scientists “implanted and prayed” that their experiments wouldn’t lead to the “horrible consequences” Knoepfler is warning against. So far, we’ve been incredibly lucky.

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