Pigs have been the most recent research focus to address the organ shortage, but among the hurdles: A sugar in pig cells, foreign to the human body, causes immediate organ rejection. The kidney for this experiment came from a gene-edited animal, engineered to eliminate that sugar and avoid an immune system attack.

Surgeons attached the pig kidney to a pair of large blood vessels outside the body of a deceased recipient so they could observe it for two days. The kidney did what it was supposed to do, filter waste and produce urine, and didn't trigger rejection.

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