Somatic cell nuclear transfer and in vitro induction of pluripotency in somatic cells by defined factors provided unambiguous evidence that the epigenetic state of terminally differentiated somatic cells is not static and can be reversed to a more primitive state (Gurdon, 2006; Jaenisch and Young, 2008; Yamanaka and Blau, 2010). Inspired by these results, stem cell biologists have recently identified approaches to directly convert fibroblasts into induced neuronal (iN) cells, indicating that direct lineage conversions are possible between very distantly related cell types (Vierbuchen and Wernig, 2011). Importantly, iN cells can also be derived from defined endodermal cells. The reprogramming process both induces neuronal properties and extinguishes prior donor cell identity, and therefore represents a complete and functional lineage switch as opposed to generation of a chimeric phenotype. Since the discovery that MyoD, a key regulatory transcription factor in the skeletal muscle lineage, can induce many features of muscle cells in fibroblasts in the late 1980s, several other examples of remarkable cell-fate changes have been observed in response to forced expression of transcriptional regulators, but until recently it was assumed
