Scientists in the United States are trying to grow human organs inside pigs.
They have injected human stem cells into pig embryos to produce human-pig embryos known as chimeras.
The embryos are part of research aimed at overcoming the worldwide shortage of transplant organs.
The team from University of California, Davis says they should look and behave like normal pigs except that one organ will be composed of human cells.
The human-pig chimeric embryos are being allowed to develop in the sows for 28 days before the pregnancies are terminated and the tissue removed for analysis.
The BBC's Panorama was given exclusive access to the research for Medicine's Big Breakthrough: Editing Your Genes.
Creating a chimera
Creating the chimeric embryos takes two stages. First, a technique known as CRISPR gene editing is used to remove DNA from a newly fertilised pig embryo that would enable the resulting foetus to grow a pancreas.
This creates a genetic "niche" or void. Then, human induced pluripotent (iPS) stem cells are injected into the embryo. The iPS cells were derived from adult cells and "dialled back" to become stem cells capable of developing into any tissue in the body.
The team at UC Davis hopes the human stem cells will take advantage of the genetic niche in the pig embryo and the resulting foetus will grow a human pancreas.