Opinion: Ending animal testing could set back xenotransplantation just as the field is poised for a breakthrough

Health secretary Robert F. Kennedy Jr. seeks to end all federally funded animal testing after concluding that “ the predictivity of animal models is very, very poor for human health outcomes .”

In November 2025, Centers for Disease Control and Prevention staff were told that the agency would be required to phase out primate studies, and they are in the process of transferring their animals to a primate sanctuary . The National Institutes of Health, the largest funder of biological sciences in the U.S., has stopped issuing funding opportunities exclusively for animal models , sending a clear message that basic science conducted in animals small and large will no longer be a priority. Of the eight NIH-funded National Primate Research Centers in the U.S., one has been shuttered , and a second is exploring the possibility of converting to an animal sanctuary .

The desire to move away from animal experimentation is based on ethical concerns and a claim that animal trials are unreliable in predicting what will happen in humans. This has been accompanied by an assertion that “ new approach methodologies ” — including organs on chips, 3D tissue cultures (organoids), and artificial-intelligence-powered computational models — can be better predictors than animal studies.

As a transplant surgeon, when I hear debates on this topic, I think about a revolution in health care we are witnessing right now. The field is xenotransplantation, or transplantation of pig organs into humans. And it’s going to change our lives — if changes to animal-testing policy don’t get in the way.

In the 1980s, scientists recognized that if a pig organ was placed in a primate (as a model for a human), it would be rejected within minutes. Old world monkeys and humans harbor a natural antibody to a carbohydrate found on all the cells of pigs and other animals below primates on the evolutionary ladder. Multiple researchers futilely attempted to devise strategies to work around this antibody, to no avail.

Then, in 1996, along came the cloning of Dolly the sheep. This breakthrough represented the first quantum advance in the history of xenotransplantation, making it possible to eventually generate a pig without this carbohydrate (termed knockout pig).

This spurred an influx of money from industry, with multiple companies looking for investment opportunities that would take advantage of cloning in both the U.K. and the U.S. Expectations were high, with predictions that xenotransplantation would become a reality in just a few years.

But it wasn’t to be. By 2004 the industry money was gone because of three challenges.

First, significant protests by animal rights activists, including the vandalizing of investigators’ homes and laboratories, drove efforts at xenotransplantation out of the U.K.

Second, the discovery of an endogenous retrovirus in the genome of pig cells raised concerns about a potential xenovirus that could become active after transplanting pig organs into immunosuppressed humans.

Third, the outcomes of the transplantation of pig organs into primates, the only model where it was/is possible to test how long a pig xenotransplant could be expected to survive in a human, weren’t improving quickly enough to achieve the companies’ aggressive timelines. Despite successfully utilizing gene therapy and cloning technology to knock the offending carbohydrate out of pig cells in 2003, kidneys and hearts could last only a few months when transplanted into primates before they were rejected.

If it weren’t for government investment in continued animal research, xenotransplantation might have ended in 2004. But a handful of committed researchers refused to give up on the field. They secured enough funding with government grants to continue the incremental advances in gene editing and identification of targets that might eventually make it worthwhile to reengage with industry, doing their work in mice and rats, pigs, and primates. Gene editing improved significantly with the development of new techniques. Novel carbohydrates on the cells of pigs were identified and deleted, and experiments led to the addition of genes in the pigs that could further modify the rejection response. Outcomes of transplants into primates slowly began to improve.

Then in 2012, the second quantum advance in the history of xenotransplantation was realized when CRISPR-Cas9 was demonstrated as a gene-editing tool. This revolutionary discovery simplified gene-editing and made it realistic to generate complex gene-edited animals rapidly and efficiently.

Once again industry money came looking for innovations that would utilize this earth-shattering technology, and xenotransplantation fit the bill. Shortly thereafter multiple animals were generated, including the United Therapeutics pig with 10 gene edits, and the eGenesis pig with 69 gene edits, the most edited large animal ever created.

In the 1990s survival of xenografts transplanted from pigs to primates could be me…