The compaction of DNA in somatic cells is thought to be inhibitory to reprogramming. The first level of DNA compaction is defined by the wrapping of DNA around nucleosomes [26]. The presence of nucleosomes can prevent binding of certain transcriptional regulators, for example to DNA binding sites and, in particular, to large DNA recognition motifs. Therefore, efficient reprogramming requires mobilisation and remodelling of nucleosomes to allow transcriptional regulators to gain access to their genomic targets [27]. Consistent with this, most of the factors with the ability to promote access to gene regulatory regions have been found to be able to increase reprogramming efficiencies (Table 1) [28].
