Scientists have created an infant ‘chimeric’ monkey by injecting a monkey embryo with stem cells from a genetically distinct donor embryo. The resulting animal is the first live-born chimeric primate to have a high proportion of cells originating from donor stem cells. However, the monkey had to be euthanized when it was only ten days old, due to health issues, thereby raising ethical concerns. DOI: 10.1016/j.cell.2023.10.005