The idea of solving the human organ shortage with pigs has tantalized surgeons for decades. More than 117,000 Americans are currently on a transplant wait-list in the U.S., according to federal figures, and 22 people die every day awaiting a match.

Pig organs are similar in size and function to our own, and people are less squeamish about harvesting body parts from an animal raised for meat than they would be about a primate’s. Yet one major hurdle that has continued to vex any such cross-species transplants, or xenotransplants, has been the threat of transmitting viruses that can infect people and pigs alike: The latter’s genome includes 25 so-called retroviruses that apparently do nothing to porkers but might transmit diseases to people—especially immune-compromised transplant patients.

That concern, particularly amid the HIV epidemic, has helped stall such research for the past couple decades (with the exception of pig heart valves that are used in humans—dead tissue that doesn’t pose the same transmission risks). Recent gene editing advances, however, are rejuvenating interest in pig-to-human transplants.

Today scientists in Massachusetts announced that by using the CRISPR–Cas9 gene-editing system they were able to inactivate all 25 viruses in the pig genome, yielding seemingly healthy piglets and moving research one step closer to a future of xenotransplantation. “Our animal is probably the most [genetically] modified animal on the Earth,” says Luhan Yang, co-founder and chief science officer of eGenesis, the Cambridge, Mass.–based start-up that led the research. “We are pushing the envelope of technology day by day. I think that such innovation is required to tackle as challenging a problem as xenotransplantation.”

At four months old—roughly the age the pig would need to be for its organs to be large enough to use in people—the animals seem perfectly normal, says George Church, a Harvard Medical School geneticist who co-founded eGenesis and is a co-author of the paper. “It’s a very, very nice piece of work,” says Joseph Tector, a transplant surgeon and professor of surgery at the University of Alabama at Birmingham School of Medicine, who was not involved in the research, published in today’s Science.

Church says he was surprised the piglets turned out to be so healthy. CRISPR can be toxic to cells because it causes breaks in DNA strands, which can lead cells to self-destruct. What’s more, retroviruses replicate by inserting a copy of their genome into their host’s so those viruses may have been part of the pig genome for the roughly 25 million years that pig species have existed. As a result, Church had wondered if they play an essential role in the pig’s survival and whether the animals could develop properly without them.

Another pleasant surprise, Church adds, is that the piglets did not get reinfected with the viruses in the womb. “I generally hesitate to say we’ve solved a two-decade-old problem, but in this case, we have,” he notes. So far the team has only made female pigs, raised in a lab. They are now repeating the process to engineer male pigs, which Church says he doesn’t expect to be any more complicated.

Pig Parts for People

The next stage of the research, Church and Yang say, will be to essentially “humanize” the pigs—modifying them enough that their organs can function in the human body. This involves immunological changes as well as making the tissues compatible and fixing blood-clotting issues. They have already begun such work and are writing it up for submission to a peer-reviewed journal, Church adds.

Other teams, including Tector’s in Alabama, are working along a similar path, hoping to get the pig parts ready to be tested in the first people within the next two to three years. Researchers expect to start by transplanting kidneys, where the human waiting list is the longest, followed by other organs like the heart and liver; pancreatic islet cells to combat type 1 diabetes; skin; and corneas.

Studying the eGenesis-edited pigs will also give researchers the opportunity to see whether editing a significant number of genes with CRISPR causes any long-term problems in mammals. Pigs are the biggest animals that have undergone CRISPR, he says, and he wants to see what happens when they are allowed to “grow to a ripe old age” of over 20. There has been some speculation that CRISPR might lead to cancers, but that has not been adequately tested, he says.

Whether or not pig retroviruses would truly pose a risk of causing disease in humans remains controversial. In their new work the Yang team performed experiments confirming that pig retroviruses can infect human cells—just as another retrovirus, HIV, does with people. In a lab dish the pig viruses infected human cells, and those infected cells were able to infect other human cells that had not been directly exposed to pig cells.

But other researchers say the risk of infecting humans with pig retroviruses is not that clear and that, on balance, unnecessarily editing the pig genes would add to the complexity and cost of a xenotransplant. Tector says his own team stopped worrying about the viruses years ago, because it is not clear whether the U.S. Food and Drug Administration will require the viruses to be removed prior to transplantation. And eGenesis’s lab tests did not prove the viruses would be a risk to patients, says Muhammad Mansoor Mohiuddin, a professor of surgery and director of Xenoheart Transplantation at the University of Maryland School of Medicine, who was not part of the new study. The viruses’ ability to infect cell lines is not enough to be of concern, he says. “I fail to understand the significance of this [infectivity] unless it is shown that it can cause some kind of disease.”

Still, Tector says, if the FDA does require the viruses to be removed, then the eGenesis team’s approach will be useful. “If you need to knock [these viruses] out, this is the way to do it, no question,” he says.