cells, whether derived from human embryos or adult tissues, are able to develop into specialized tissues, and seeks to harness this potential for tissue-replacement therapies that will restore lost function in damaged organs.

The list of diseases and injuries cited as potential targets of stem cell therapy reveals, in large measure, why stem cells offer so much hope for revolutionary advances in medicine (Table 1). Many of them—such as Parkinson’s disease, diabetes, heart disease, Alzheimer’s disease, and spinal cord injury—have few or no treatment options, so millions of Americans are currently looking for cures.

The hope of using stem cells to produce regenerative therapies poses fundamental questions: Do human ESCs hold all the clinical promise attributed to them? Is realization of that promise imminent? Do stem cells from all sources have the same abilities? What is their potential for regenerative medicine?