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Human-animal chimeric embryos—organisms created using cells from two or more species—have the potential to change how researchers study disease and generate organs and tissues for human transplants. One day, scientists have proposed, it may be possible for someone with, say, pancreatic cancer to have their stem cells injected into a modified swine embryo lacking its own pancreas so it can grow the human organ for donation.

Already, human-animal chimeric embryos (HACEs) have been created using human cells injected into pigs, sheep, micerats, and monkeys, although none in the US have been brought to term. In fact, their very existence is ethically contentious. What happens, for example, if scientists were to grow a human brain in an animal, blurring the line between species? 

In response to ethical, social, and legal concerns, the National Institutes of Health (NIH) issued a moratorium on funding for HACE research in 2015 pending the development of a new set of regulatory guidelines. While research continues in other countries—and even in the US, through collaborations with foreign researchers and private funding—the NIH... see more