As transcription factors play key determining roles in cell fate specification, we hypothesized that forced expression of a combination of such factors may be sufficient to directly convert mouse fibroblasts into neuronal cells (Vierbuchen et al., 2010). Following lentiviral expression of 19 candidate genes we detected cells with neuronal morphologies and expression of neuronal markers suggesting such a conversion may indeed be possible. A systematic evaluation of different combinations revealed that the 5 transcription factors Ascl1, Brn2, Olig2, Zic1 and Myt1l are the most critical genes for this process. Out of those 5 factors the pool of Brn2, Ascl1 and Myt1l (BAM) was shown to be sufficient to induce neuronal cells that unexpectedly not only exhibited molecular but all principal functional properties of neurons (Figure 1). Even more surprising, the conversion efficiency of embryonic fibroblasts was estimated to be close to 20% within 2 weeks indicating that the conversion towards neuronal fates is substantially faster and more efficient than iPS cell formation. Also in contrast to induction of pluripotency, the iN cell reprogramming does not require cell proliferation, which is
