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 proliferating cell hybrids, progression through the cell cycle causes the nuclei to fuse and give rise to synkaryons, which we do not discuss here. In heterokaryons, the nucleus of one donor cell is induced to express genes characteristic of the other donor cell, thereby providing an opportunity to investigate the mechanism of reprogramming. The cells fused can be of different species or differentiation state. For example, mouse ES cells can be fused to human fibroblasts [9]. Pluripotency can be induced in somatic cells by overexpression of a few transcription factors, originally Oct4, Sox2 (both of which are required for pluripotency), Klf4 and c-Myc (Figure 1d) [11]. The induced pluripotent stem (iPS) cells obtained have been well reviewed by others [12,13]. However, regardless of the system used, the proportion of nuclei or cells that are reprogrammed to new cell types is always low. This shows the resistance of somatic cells to reprogramming and reflects the stability of the differentiated state. Here,
