Scientists who dream of transplanting organs from pigs into people have long faced a nagging question: What about PERVs? Remnants of ancient viral infections, genes from porcine endogenous retroviruses—known by their unfortunate acronym—are scattered throughout the pig genome, and could infect a person who one day receives a pig’s heart, lung, or kidney as a replacement or temporary organ. Now, a U.S. company aiming to grow transplant-friendly pig organs reports that it has crossed these viruses off the list of concerns. Using CRISPR gene editing, researchers from the company and several academic labs created dozens of apparently healthy pigs with no trace of PERV genes.

“If this is correct, it’s a great achievement,” says virologist Joachim Denner of the Robert Koch Institute in Berlin who has studied the mysterious PERV sequences. Scientists still don’t know whether the viral particles they produce can infect humans at all, he notes, much less whether they would cause disease if they did. And even with PERVs off the table, pigs will require other modifications so that their organs won’t be rejected by the human immune system or cause other harms. Still, Denner says, “If it is possible to knock [PERVs] out, you should do it.”

A shortage of human organs creates long waitlists for vital transplants; in the United States, about 22 people needing various organs die every day while they wait. Pig organs, meanwhile, can grow to a conveniently human size. The concern about PERVs has been hard to dismiss, however, especially because studies have shown that the viruses can infect human cells in a dish.

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