Human tissues vary in their ability to heal and regenerate. The nervous system has weak powers of regeneration, while the skin is quick to make new cells for repair. The heart is the most important muscle in the body and yet has feeble regenerative capabilities. Recent discoveries concerning the location and characteristics of adult stem cells have increased our understanding of regeneration. Insulin-like growth factor 1 (IGF1) is an example of an important stem cell communication molecule. If the activity of the growth factor is experimentally enhanced, muscle regeneration improves. From the Series:HHMI’s Understanding Biomedical Research Series