Embryonic stem cells can potentially revolutionize the way we treat disease, providing a source of tissue to replace or repair a huge variety of disorders. In the US, however, funding of stem cell research has been limited by policy decisions and, once those decisions were reversed, a court ruling. In the intervening years, researchers developed what appeared to be a promising alternative, a technique that could push any adult cell into a state that appeared to be indistinguishable from an embryonic one. Unfortunately, researchers have now looked a bit more closely at these induced pluripotent stem cells (iPSCs), and found that they are distinct from embryonic ones, in part because they retain the imprint of their former, mature state.

Embryonic stem cells (ESCs), like the earliest cells of the embryo, have the ability to develop into any adult cell type: cardiac, neural, pancreatic, etc. An adult cell, including adult stem cells like the ones in bone marrow, typically is committed to one, or at least a limited number of fates. This process of commitment is partly controlled by the proteins they express, which are relatively easy to change. But only partly. As cells go through the process of development and become committed to role in a specific organ, their DNA is chemically modified to lock this developmental decision in place.

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Nonembryonic stem cells can’t forget their past