Every day, 22 people in America die while waiting for an organ transplant. But when scientists can grow replacement livers or kidneys or pancreases inside of animal hosts, medicine’s organ shortage may end. That’s the hope anyway—and this week there’s more reason to hope than ever that it might become reality. The key to producing human organs in other animals is the chimera, a mixture of cells from more than one species growing together as a single animal.

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