Despite the excitement surrounding stem cells' potential to perhaps cure disease or unlock the secrets of development, a fundamental question remains: What, exactly, are stem cells? In common parlance, they have been defined as cells that can both renew themselves and give rise to more specialized daughter cells. But that is a functional definition, akin to saying that a car is a movable machine on four wheels. Scientists are keen to get under the hood and see which genes drive stem cells' engine of renewal. Although researchers have identified a few genes that seem to play a role, the key molecular switches remain a mystery.
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