Cellular plasticity is a major focus of investigation in developmental biology. The recent
discovery that induced neuronal (iN) cells can be generated from mouse and human
fibroblasts by expression of defined transcription factors suggested that cell fate plasticity is
much wider than previously anticipated. In this review, we summarize the most recent
developments in this nascent field and suggest criteria to help define and categorize iN cells
that take into account the complexity of neuronal identity.