Researchers have discovered a key protein that controls how stem cells “choose” to become either skeletal muscle cells that move limbs, or smooth muscle cells that support blood vessels, according to a study published today in the Proceedings of the National Academy of Sciences (PNAS). The results not only provide insight into the development of muscle types in the human fetus, but also suggest new ways to treat atherosclerosis and cancer, diseases that involve the creation of new blood vessels from stem cell reserves that would otherwise replace worn out skeletal muscle. The newly discovered mechanism also suggests that some current cancer treatments may weaken muscle, and that physician researchers should start watching to see if a previously undetected side effect exists.