Another aspect of resistance to reprogramming comes from the phenomenon of epigenetic memory, when different cell types are compared. In both nuclear transfer to egg experiments [23] and induced pluripotency work [24,25], reprogrammed nuclei and cells show persistent expression of genes that were active in donor cells, even though such genes are not normally transcribed in the derived cell types. In these cases, active genes resist a switch off after nuclear transfer or induced pluripotency, and this resistance can continue for numerous cell divisions.
