In reprogramming experiments when new cell types are not formed (Xenopus oocytes and heterokaryons), the transcription of pluripotency and other genes is used as a measure of successful reprogramming (Figure 1). In nuclei transplanted to Xenopus oocytes, the rate of transcription of such genes increases greatly from an undetectable level in donor cells to 1200 (or 170) new transcripts per gene per day for Sox2 (or Oct4) [5]. The mechanism of this transcriptional activation is known to be related to an exceptionally high content of transcriptional components in Xenopus oocytes. This includes enough polymerase II for the transcription of over 10 000 somatic nuclei [76,77], as happens when normal Xenopus embryos reach the stage of transcriptional activation (the blastula stage) [78]. All polymerase II in the blastula is thought to be derived from the oocyte content [77]. Histone H3.3 is closely associated with active transcription [79] and is exceptionally abundant in oocytes (G. Almouzni, personal communication). Also, a high content of polymerised actin is characteristic of the oocyte germinal vesicle; it is present in somatic nuclei that are reprogrammed by
