of cell division. We conclude that resistance to reprogramming in nuclear transfer experiments is caused, at least in part, by incomplete chromatin decondensation, incomplete removal of differentiation chromatin marks and, hence, by incomplete transcriptional activation. As cells differentiate, they progressively acquire more and more epigenetic marks that restrict reprogramming. Although oocytes are endowed with components that promote nuclear reprogramming, it may be that the process of cell differentiation progressively compacts the chromatin of specialised cells, in particular that of quiescent genes, so that access to important genes is a slow process.
