http://www.bbc.com/news/health-40886600

Tests identified 25 Pervs - porcine endogenous retroviruses - hidden in the pig's genetic code.

Experiments mixing human and pig cells together showed those viruses could escape to infect human tissues.

But the researchers then used the game-changing gene-editing technology Crispr to delete the 25 Pervs.

It then took cloning technology, the same used to create Dolly the sheep, to place the genetic material from those cells into a pig's egg and create embryos.

The complex process is inefficient, but 37 healthy piglets have been born.

"These are the first Perv-free pigs," Dr Luhan Yang, one of the researchers from Harvard University and the spinout company eGenesis, told the BBC News website.

They were also "the most genetically modified [animals] in terms of the number of modifications", he said.

If xenotransplantation - using organs from other species - works, then it has the potential to alleviate long waits for a transplant.