Building on the generic properties of neurons, we would like to propose criteria to define a fully reprogrammed iN cell (Figure 3). Similar to the maturation process of cultured primary neurons, reprogramming fibroblasts gradually and asynchronously acquire the full set of neuronal properties over time (Vierbuchen et al., 2010). Therefore, at any given time there will be a heterogeneous population containing iN cells that are reprogrammed to different degrees and thus display a range of neuronal features. Figure 3 shows such properties in the order of stringency, which is roughly also the order of appearance in reprogramming iN cells or neurons during development. These properties, therefore, can serve as criteria to help define the degree of reprogramming/ maturation stage of iN cells.
