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. Mammalian muscle cells are intermediate in their ability to regenerate. Human muscle can regenerate in response to minor wounds and normal wear and tear, but humans will not grow a new bicep, for example, in response to amputation. The heart is the most important muscle in the body and yet has feeble regenerative capabilities. Research into the wholesale production of new replacement organs and limbs is in its infancy, but research into enhancing normal levels of regeneration is progressing rapidly. Recent discoveries concerning the location and characteristics of adult stem cells and the signals that wounded tissue produces to activate 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.