A gene-editing method could one day make pig organs suitable for use in people, scientists say.
Prof George Church and colleagues used a technique called Crispr to alter the DNA of pig cells to create a better match for humans.
The early work, in the journal Science, aims to address concerns about rejection and infection by viruses embedded in pig DNA.
If successful, it could be an answer to the shortage of human donor organs.
Years more research is needed before genetically modified pigs could be bred to grow organs for people.
Crispr is a relatively new scientific tool that lets scientists snip and play around with the code of life - DNA.
Prof Church, from Harvard University, used it to inactivate a retrovirus present in the pig cell line.
This porcine endogenous retrovirus is potentially risky because it can infect human cells - at least in the lab.
In tests on early pig embryos, Prof Church was able to eliminate all 62 copies of porcine endogenous retroviruses from the pig cells using Crispr.
Next, he checked if the modified pig cells would still easily pass the retrovirus on to human cells. They did not, although there was still a small amount of transmission.
Prof Church says the discovery holds great promise for using animal organs in people - what doctors call xenotransplantation.
Prof Church, who part-owns a company that wants to develop modified pigs to grow organs, said: "It was kind of cool from two stand points.
"One is it set a record for Crispr or for any genetic modification of an animal, and it took away what was considered the most perplexing problem to be solved in the xenotransplantation field.
"With immune tolerance, that completely changes the landscape as well.
"These two things, immune tolerance and now getting rid of all the retroviruses, means we have a clear path."
Dr Sarah Chan, an expert from the University of Edinburgh, said: "Even once the scientific and safety issues have been addressed, we should be mindful of the possible cultural concerns and societal impacts associated with more widespread use of pig organs for human transplantation.
"Nonetheless, the results of the study are valuable both as a proof of principle and a potential step towards therapeutic advances in this area of much-needed research."