Scientists successfully transplanted two kidneys from a genetically modified pig into a human recipient and found that the organs produced urine and were not rejected during the days-long experiment.

The procedure was performed in a brain-dead patient who was a registered organ donor and whose family authorized the research, according to the new study, published Thursday (Jan. 20) in the American Journal of Transplantation. The research team intends to eventually transplant pig kidneys into living patients, in formal clinical trials — but first the team wanted to address some critical safety questions.

In the new study, the researchers transplanted not one, but two pig kidneys inside a recipient’s body, where kidneys would be placed during a conventional human-to-human transplantation…From the procurement of the pig’s kidneys to the surgery itself, the study followed the exact same procedure that the team will use in a future clinical trial, Dr. Jayme Locke, lead surgeon said…

Most of these genetic modifications are intended to reduce the risk of a transplant being rejected by the human body. For instance, the modified pigs lack three genes that each code for specific carbohydrates; in the human body, these carb molecules can set off an aggressive immune reaction. The donor pigs also lack a gene that codes for a specific growth hormone receptor, and without this receptor, the pigs’ organs should stop growing once transplanted into a person.

Finally, the pigs carry six extra genes plucked from the human genome: four to help make each pig’s organs appear more familiar to the human immune system and two to prevent the formation of blood clots.

Researchers are getting better and better at this – all the time. Sooner or later, full-blown ethics reviews will have to allow for inter-species transplants.