In recent months there has been a wave of reports published describing various methods to generate iN cells of various sorts. Given the infancy of the field, different criteria were applied to define converted neuronal cells, complicating the direct comparison of the different approaches. Consistent standards would be helpful, and we would therefore like to suggest here a panel of criteria that can be used to define iN cells of various degrees of reprogramming. First, we propose the term “induced neuronal cells (iN cells)” as opposed to “induced neurons”, to contrast reprogrammed cells with brain-derived cells. Second, we propose that the term “iN cell” should only be endorsed when the extent of reprogramming can be documented as being reasonably complete. In a nutshell, we believe fully reprogrammed iN cells should have a distinct neuronal morphology, express neuron-specific gene products, and 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;
