Its age-reversal plans build on tantalizing clues seen in simple organisms like worms and flies. Tweaking their genes can increase their life spans by double or better. Other research has shown that giving old mice blood transfusions from young ones can restore some biomarkers to youthful levels.

“We have already done a bunch of trials in mice and we are doing some in dogs, and then we’ll move on to humans,” Church told the podcaster Rob Reid earlier this year. The company’s other founders, CEO Daniel Oliver and science lead Noah Davidsohn, a postdoc in Church’s sprawling Boston lab, declined to be interviewed for this article.

The company’s efforts to keep its activities out of the press make it unclear how many dogs it has treated so far. In a document provided by a West Coast veterinarian, dated last June, Rejuvenate said its gene therapy had been tested on four beagles with Tufts Veterinary School in Boston. It is unclear whether wider tests are under way.