On Thursday, the National Institutes of Health announced that it was revising the rules that govern its funding of stem cell research. The rules focus on cases where human stem cells are introduced into embryonic animals, creating an embryo that’s a mixture of human and animal tissues. While the rules would lift a blanket moratorium on funding for this research, they’d also tighten the regulations that were in place prior to the moratorium.

An animal that’s a mixture of two different organisms is called a chimaera. While they’re named after mythological beasts, creating them is rather run-of-the-mill in modern research, where chimaeras between different mouse strains are an essential part of knocking out genes to study the effects. Human-mouse chimaeras are also quite common, as we inject human tumor cells into mice to study cancer and replace the mouse immune system with a human one in order to study diseases like AIDS.

Some stem cell research would be similar in nature. For example, if you wanted to determine if it is safe and effective to use stem cells to repair cardiac injury in humans, a reasonable first step would be to see what happens when you inject human stem cells into an adult mouse that has a damaged heart.