Embryonic stem cells have the ability to differentiate into any type of cell in the human body. Incorporating these highly morphological cells into an animal embryo or brain opens up amazing scientific possibilities and unthinkable ethical quandaries, such as a human brain trapped in a mouse's body, or a human baby with mice for parents.
No scientist working with chimeras apparently wishes to create such nightmarish animals. But just to be on the safe side, the National Academies made chimeras a prominent part of a larger report outlining guidelines for embryonic stem-cell research.
The guidelines, which are voluntary, say scientists should never implant human embryonic stem cells into non-human primates, such as chimpanzees.