There are several issues which are under intensive discussion, like the influence of DNA and H3K9 methylation on the binding of the reprogramming factors in the early phase of the reprogramming process. The hypermethylation status of the Nanog and Oct4 promoters in the starting phase which gain active chromatin modifications in the late phase of the reprogramming is very well analyzed examples [25, 26]. There are other very important points which have to be considered with respect to reprogramming such as possible transition between reprogramming steps or the status of the X chromosome and its inactivation while reprogramming, all of which are reviewed in [27].
