exhibit the two principal functional properties of neurons: action potentials and synaptic transmission. This is equivalent to the validation of in vitro-generated neurons from neural or embryonic stem cells in previous work (Song et al., 2002; Vicario-Abejon et al., 2002; Wernig et al., 2004). Cells exhibiting only a subset of these properties should be termed “partially-reprogrammed iN cells”. Since the reprogramming process mimics neuronal maturation the term “immature iN cells” could be used alternatively. We would like to point out though that it is difficult to distinguish these two conceptionally very different interpretations.
