Mature organisms have stem cells of various sorts, called adult stem cells. Adult stem cells are an essential source of cells for healing and regeneration in response to injury. Some animals, such as sea stars, newts, and flatworms, are capable of dramatic feats of regeneration, producing replacement limbs, eyes, or most of a body. It is an evolutionary puzzle why mammals have more limited powers of regeneration. Researchers are interested in pinpointing where adult stem cells reside and in understanding how flexible adult stem cells are in their ability to produce divergent cells such as muscle and red blood cells. From the Series:HHMI’s Understanding Biomedical Research Series