Core Concept: Chimeras keep courting controversy.

Last autumn, the US National Institutes of Health announced it would not fund any research that entails introducing human pluripotent stem cells into early embryos of another species, including laboratory stalwarts, such as mice, or larger animals like pigs. The agency said it wanted a chance to conduct a formal policy review on these interspecies “chimeras.” The backlash from the scientific community was swift. A group of leading stem cell experts wasted no time in condemning the move in Science as “a threat to progress” (1). “It was a ham-handed approach,” says Christopher Scott, a bioethicist at Baylor College of Medicine in Houston, and one of 11 authors of the Science commentary. “That’s a really terrible way to do science policy because it just creates panic and makes scientists uncertain about their lines of inquiry.” In August, the NIH reversed course on the ban and proposed allowing funding for studies involving chimeras, with some added provisos. Still, according to Scott, the 10.5-month funding hiatus was long enough to “put at risk some big and important areas of research” in developmental biology, disease modeling, and regenerative medicine. It’s the latest twist in a decades-long saga for the organismal and cellular contrivances that some consider to be fraught with ethical barriers, and others see as a practical means toward key research insights and disease cures. “There was always controversy,” says Esmail Zanjani, an experimental hematologist at the University of Nevada, who developed the world’s first human–sheep chimeras in the mid-2000s. Already, since 2009, the NIH has prohibited funding for experiments that put human stem cells into the embryos of our close primate relatives, or studies in which chimeras of any species that have human reproductive cells are allowed to breed. These rules will likely stand with minor tweaks, although the agency is considering an additional layer of scrutiny.