Different systems have been used to reprogram cells (Figure 1). These include nuclear transfer to eggs and oocytes, cell fusion and overexpression of transcription factors. The nucleus of a specialised cell can be reprogrammed by somatic cell nuclear transfer (SCNT) to an enucleated egg (also called metaphase II oocyte; [1–3] but see also [4]). In this case, a somatic cell nucleus is reprogrammed by the egg to behave like the nucleus of an embryonic cell, and cells of the resulting embryo are pluripotent and able to differentiate into many, and sometimes all, cell types unrelated to the original donor nucleus (Figure 1a). The transcriptional state of somatic cell nuclei can also be reprogrammed by nuclear transfer to Xenopus meiotic prophase I oocytes (Figure 1b) [5]. Another route is to fuse two cells from different origins in such a way that the two nuclei of different cell types occupy the same cytoplasm; such fused cells form heterokaryons and cell hybrids (Figure 1c) [6–10]. In heterokaryons, the nuclei remain as separate entities within a common cytoplasm for a few days [8]. In
