The reprogramming of somatic cells has been extensively investigated. Successful studies have yielded the generation of cloned animals from frog and sheep somatic cells. The somatic cells were fused with enucleated oocyte in those studies, which indicated the existence of a reprogramming factor in the oocyte. Embryonic stem (ES) cells derived from early embryonic tissue have similar activity, and can reprogram somatic cells by cell fusion. Reprogramming was then accomplished with defined factors (Oct3/4, Sox2, Klf4, and c-Myc), which are expressed abundantly in ES cells and pluripotent stem cells emerged. These artificial cells were termed induced pluripotent stem (iPS) cells
they have similar morphology, proliferation, and gene expression profile to those of ES cells. These in vitro reprogrammed cells attract a lot of attention because they have potential to supply patient-specific pluripotent stem cells to many fields, such as the study of disease pathogenesis, drug discovery, toxicology, and someday maybe even cell therapy.