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Embryonic stem (ES) cells, the undifferentiated cells of early embryos are established as permanent lines (1,2) and are characterized by their self-renewal capacity and the ability to retain their developmental capacity in vivo (3) and in vitro (4–6). The pluripotent properties of ES cells are the basis of gene targeting technologies used to create mutant mouse strains with inactivated genes by homologous recombination (7).