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For years now, biologists have been practicing a kind of time travel. You can take a speck of human skin, and with the right genetic tweaking, turn back its inner clock until it becomes its embryonic self, stripped of its identity and ready to become just about any human body part at all. Since the method was published in 2006, transforming adult cells into stem cells has allowed all sorts of advances. Researchers can grow organs in dishes. They can replicate what happens in the womb without the regulatory headache of acquiring fetal tissue. It became an everyday tool at lab benches around the world and won a Nobel Prize.

But for David Sinclair’s purposes, that wasn’t good enough. His interest was in reversing the slings and arrows of old age, using that genetic time machine to create something that would actually be therapeutic. When cells were rewound to an embryo-like state, they did what embryonic cells do: divide like crazy. Outside the intricate control of the prenatal environment, that gave rise to cancer. The mice used in such experiments died within days. “We wanted to take the age of a tissue backward, but find a way to stop it from going too far back,” said Sinclair, a professor of genetics at Harvard Medical School.

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