English: (A) A Waddington-inspired schematic of the epigenetic landscape. Culture conditions will promote the self-renewal of a pluripotent cell, maintaining it in a shallow well at the top of a cellular potential hill. When allowed to differentiate, this cell will “roll” down the hill into one of many terminally-differentiated fates at lower potential. (B) A closer look at the path a pluripotent cell might take as it differentiates into a neuron, passing through a number of intermediate progenitor states of varying stability on the way. The line in (B) represents a slice through the surface shown in (A). (C) The process of direct reprogramming, like chemical catalyst, implicates a restructuring of the epigenetic landscape. Introduction of the transcription factor cocktail destabilizes the fibroblast identity while stabilizing the transition state. Because the retroviruses are shut down in the iPS cells, however, the potential of the pluripotent state remains unchanged.