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 reprogramming [21]. We found that the addition of NeuroD1 greatly facilitated the neuronal induction in human fibroblasts. Those human iN cells expressed multiple pan neuronal marks, exhibit spontaneous action potentials and can be integrated into existing mouse cortical neuronal networks [21]. Yoo et al examined whether brain specific microRNAs (miR-9/9* and 124) could have instructive roles in induced neuron reprogramming [23]. Remarkably, the authors showed that overexpression of miR-9/9* and miR-124 are sufficient to generate MAP2-positive iN cells, however co-introduction of the three transcription factors NeuroD2, Myt1l and Ascl1 was required to generate functional iN cells. Albeit rare, postsynaptic activity could be detected even in the absence of co-cultured glial cells. Along the same vein, Ambasudhan et. al. reported miR-124 together with two other transcription factors, Brn2 and Myt1l, in defined conditions are enough to convert human postnatal and adult fibroblasts into mostly excitatory neuronal cells [22]. Remarkably, in their transcription factor combination, the authors did not use any of
