The National Institutes of Health (NIH) is preparing to end a year-old ban on publicly-funded scientific research into human-animal hybrids, paving the way for scientists to create new, fascinating, and potentially horrifying specimens.

The proposed rules would allow scientists to receive federal funds for the creation of chimeras, embryos that use human stem cells to combine human and animal traits.

The NIH originally placed a moratorium on funding such research in September 2015, citing potential ethical concerns with the research. Now, the agency says it is implementing various regulatory controls to avoid major potential transgressions.

The stakes are potentially quite high. For instance, some scientists and ethicists have raised the possibility that chimera research could result in animals that possess substantially human brains, or animals that have human sperm and eggs and could therefore create human offspring when breeding.

A poll released last week by Pew Research indicated that Americans are currently more worried than enthusiastic about using gene editing to create modified humans.

But many scientists have still been ardent advocates for chimeras, arguing they could help cure human diseases or provide other major scientific advancements. For instance, some scientists are hoping to grow human organs in pigs, in the hopes of providing transplants.

To avoid any severe consequences, the NIH is proposing several new rules. For instance, the new rules would very strictly limit how human cells may be combined with non-human primate cells, due to those animals’ close genetic proximity to humans. Certain chimeras, such as those giving animals human brain cells or sperm cells, would have to obtain approval from a special committee to avoid unwanted developments. For instance, animals with human sex cells would have to be kept from breeding.

“At the end of the day, we want to make sure this research progresses because its very important to our understanding of disease,” NIH associate science policy director Carrie Wolinetz told NPR.

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