Our group found that functional neurons can be generated from a non-ectodermal cell type demonstrating that direct reprogramming is possible between two different germ layers [6]. Starting from a pool of 19 candidate transcription factors, systematic enriched iterations of combinations narrowed the candidates to just three active transcription factors: Brn2, Ascl1, and Myt1l (the “BAM” pool of factors). Those iN cells exhibited stereotypical neuronal morphology, expressed multiple pan-neuronal and subtype specific markers such as b-III-tubulin, Map2, NeuN, Tau, vGluT1, Tbr1, and synaptic markers when cultured for longer time periods. Moreover, the cells also exhibited the two functional principal properties of neurons: the ability to fire action potentials and synapse formation. We demonstrated that fibroblast-derived iN cells could form synapses within themselves, and could also integrate into pre-existing cortical neuronal networks where they exhibited short term synaptic facilitation and depression [6]. We and several other groups extended these findings to human fibroblasts [21–23]. Since the BAM combination of factors yielded only immature-looking iN cells, we screened an additional set of 20 factors on top of the BAM factors to improve neuronal
