Nearly a year ago, the National Institutes of Health (NIH) decided to stop funding research in a promising area of stem cell science. Scientists eager to find new ways to generate desperately needed human organs or tissues to replace damaged or diseased ones have been excited by the possibility of inserting a specific type of human stem cell, which has the potential to turn into any type of tissue, into animals, where they could develop and eventually be transplanted into humans. The process creates human-animal chimeras, similar to those highlighted in science fiction, where humans take on animal-like features and animals take on human characteristics. The NIH decided to stop funding such research over concerns about the unpredictable consequences and ethically unresolved questions that such chimeras raise.

But the agency now says that it will lift the ban and put in place a review process that would require two types of chimera studies to get further review. These include experiments in which human stem cells are added to very early embryos of other animals, and studies in which the human stem cells are injected into the brains of mammals other than rats and mice.

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The decision could reinvigorate the next phase of stem cell research, which has always been plagued by controversy and ethical concerns over how such powerful cells, which can theoretically seed new human beings, are handled. The NIH will continue to ban studies in which chimeras are allowed to reproduce, or in which human stem cells are injected into early embryos of our closely related primates.

But for researchers like Juan Carlos Izpisua Belmonte at the Salk Institute, it represents an opportunity. Belmonte is studying ways to produce human tissues for transplant but has been working a collaborators in Spain since his studies are not permitted in the U.S. “The possibility of NIH supporting our current program of research will enhance and accelerate our goals toward function integration of patient derived cells into a developing embryo from a different species,” he said to TIME in an email. Such studies will not only potentially provide new sources of human tissues for transplant but also help expose some of the still mysterious ways that diseases like cancer develop.

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