The human-pig hybrid that made headlines was a chimeric embryo containing both human and pig cells, grown in a lab as part of efforts to eventually produce transplantable human organs inside pigs. That line of research has advanced significantly since the first experiments, with a Chinese team growing embryonic kidneys that were up to 70% human cells inside pig embryos in 2023. Meanwhile, a parallel and faster-moving approach, transplanting genetically modified pig organs directly into human patients, has already reached clinical trials.
The two approaches are distinct but share the same goal: solving a chronic shortage of transplantable organs. Here’s where both stand now.
The Chimera Experiments
A chimera is an organism whose cells come from two different species. In this case, researchers inject human stem cells into early-stage pig embryos, hoping those human cells will grow into a specific organ as the embryo develops. The idea is that if you disable the pig’s ability to grow its own kidney, for example, the human stem cells will fill that gap and build a kidney made largely of human tissue.
The technique is called blastocyst complementation, and it has worked well between closely related species like rats and mice. Getting it to work between humans and pigs is far harder because the two species are separated by roughly 80 million years of evolution, and their embryos develop on very different timelines.
The most significant result so far came from the Guangzhou Institutes of Biomedicine and Health in China. Researchers used an optimized version of the technique to grow embryonic kidneys inside pig embryos that had been engineered to lack their own kidney development. They allowed five embryos to develop for 25 to 28 days inside surrogate sows. The proportion of human cells in the chimeric kidneys reached up to 70%, and human cells made up as much as 58% of the kidney’s tubular structures. These were primitive, embryonic-stage kidneys, not fully functional organs, but the result was a major proof of concept.
No chimeric embryo has been carried to term. The experiments are deliberately stopped early, both for ethical reasons and because the science isn’t yet ready to produce a full-sized, functional organ this way. Only one of seven published blastocyst complementation studies has used human donor cells in a pig host. The rest have been rat-mouse chimeras, where the biology is more forgiving.
Why Chimera Research Faces Strict Limits
The NIH historically maintained a moratorium on federal funding for research that introduces human stem cells into animal embryos, though it announced plans to lift that restriction. Even with funding available, significant guardrails remain. NIH guidelines prohibit introducing human stem cells into early-stage embryos of non-human primates. Breeding any animals that contain human cells contributing to sperm or egg development is also banned. The core concern is that human cells could migrate to the brain or reproductive organs, raising questions about the moral status of the resulting organism.
These restrictions mean chimera research progresses slowly and largely outside the United States. The Chinese experiments were able to move forward in part because of a different regulatory environment.
The Faster Track: Pig Organs in Human Patients
While chimera research aims to grow human organs inside pigs, a second approach skips that step entirely. Instead, scientists genetically modify pigs so their organs are less likely to trigger the human immune system, then transplant those organs directly into patients.
This field, called xenotransplantation, has moved into human patients. The results so far have been a mix of milestone achievements and sobering setbacks.
Pig Heart Transplants
David Bennett Sr. became the first person to receive a genetically modified pig heart in January 2022 at the University of Maryland. He survived about two months. His post-transplant course was difficult: he developed an aortic tear during surgery that required repair, suffered kidney failure needing regular dialysis, and underwent two abdominal surgeries for a suspected infection. His immune system was severely weakened, and a treatment intended to boost his antibody levels likely contained anti-pig antibodies that may have accelerated rejection. On top of that, the pig heart was found to harbor a latent pig virus (porcine cytomegalovirus) whose reactivation probably contributed to inflammation and his eventual death.
A second patient, Lawrence Faucette, received a pig heart in 2023 and survived 42 days. He also required dialysis for kidney failure, and rejection was reported as the cause of death. Fewer clinical details have been published about his case.
Pig Kidney Transplants
Pig kidneys have fared somewhat better. The longest a pig kidney has functioned in a human patient was four months and nine days, a record set by a patient named Looney. The kidney came from Revivicor, a division of United Therapeutics, and had 10 gene edits: three removed pig-specific sugar molecules on cells that trigger rejection, while others were designed to reduce blood clotting and further tame the immune response.
Separate from the sugar molecules on cell surfaces, pig DNA contains dozens of copies of ancient viruses called porcine endogenous retroviruses (PERVs) baked into the genome. These viruses can potentially infect human cells, with risks including immune suppression and tumor growth. One biotech company, eGenesis, used CRISPR to inactivate all copies of these viruses in pig cells, then cloned pigs from those edited cells to create virus-free donor animals.
Where Things Stand Now
The FDA cleared eGenesis in 2025 to begin a formal clinical trial of CRISPR-edited pig kidneys in living human patients. This marks a shift from the earlier cases, which were individual compassionate-use procedures in patients who had no other options, to structured trials that will generate systematic data on safety and effectiveness.
The chimera approach and the xenotransplant approach are on very different timelines. Xenotransplantation is already in patients and entering clinical trials. Growing fully human organs inside pig chimeras remains years away from producing anything transplantable, though the 2023 kidney result showed the underlying biology can work. Both paths are attempts to solve the same problem: more than 100,000 people in the United States alone are waiting for an organ transplant, and thousands die each year before one becomes available.