Mice live only months in the wild but can survive two to three years in the lab. Those in the latest experiment were already 124 weeks old when they got the drug—close to the end of their lives. Not only did the treated mice survive noticeably longer, according to Davidsohn, but they also scored better on measures of general health.

The amount of life extension observed is not in itself unprecedented. A US government program that tests drugs for their longevity effects has shown that several compounds, including the drug rapamycin, can prolong mouse lives by 5 to 15%.

But the mice have to take those drugs for much of their lives, whereas reprogramming has immediate effects.  “This is like you can do nothing for your whole life and still get the benefit,” says Davidsohn.

What’s next?

Rejuvenate is currently developing gene-therapy drugs for pet dogs and humans, including one designed to treat heart failure. But Davidsohn says that in the long term he believes it will be possible to rejuvenate human beings. “I wouldn’t be working on it if I didn’t believe that,” he says.

Far more information will be needed to learn exactly what changes the reprogramming genes cause in the mice, and researchers say other groups will need to repeat the experiment before they are convinced. “I’d like to see a separate group do something similar and go deeper onto what is actually happening,” says Borch Jensen.

Sebastiano says the life-extension effect reported by Rejuvenate could be due to changes in a single organ or group of cells, rather than a general mouse-wide rejuvenation effect. Among other shortfalls in its research, Rejuvenate did not carefully document which and how many cells were changed by the genetic treatment.

Several companies are now pressing forward with plans for reprogramming drugs, but they are picking recognized medical conditions and narrowing their efforts to specific organs.

Turn Bio, a company cofounded by Sebastiano, for instance, hopes to inject reprogramming factors into people’s skin to fight wrinkles or restart hair growth. Another company, Life Biosciences, is preparing to test whether reprogramming cells in the eye can treat blindness.