Stem cells are undifferentiated cells—meaning that they do not have a specific function—that are capable, under certain conditions, of producing cells that can become a specific type of tissue. Stem cells could be used to grow new hearts that could be transplanted without fear of rejection. They could be used to renew the function of injured structures like the spinal cord. They could be used as cell models for drug testing, thereby increasing the speed for finding cures. The potential benefits of stem cells have made the laboratory studies and research investigating these cells very exciting, yet controversial.