20 additional factors in combination with BAM and found that by introducing the bHLH factor NeuroD1, the generation of functional neuronal cells from human fibroblasts can be achieved. These human iN cells expressed a variety of neuronal markers including Tuj1, MAP2, NeuN, Neurofilament and Synapsin and exhibited functional neuronal properties as judged by measurement of action potentials. Moreover, when cultured with primary cortical neurons both spontaneous and evoked postsynaptic currents could be detected in these cells, demonstrating their synaptic maturation. However, the first functional synapses were found only after 5–6 weeks suggesting that the full maturation of human iN cells is a slow process. The same four factors could also convert postnatal foreskin fibroblasts into synaptically competent iN cells with comparable timing and efficiency. Like mouse iN cells, most of the human iN cells expressed mRNAs characteristic of glutamatergic neurons, such as vGLUT1 and vGLUT2. After downregulation of the exogenous transcription factors, the iN cells retained their stability, which indicated that an intrinsic program was established to maintain the newly adopted neuronal identity. The overall efficiency of generating human iN cells with four factors (2–4%) was about 10-fold lower than that of mouse with just three factors (compare to (Vierbuchen
