Embryonic stem (ES) cells are undifferentiated cells derived from early mouse embryos, which under appropriate culture conditions proliferate continuously in vitro. ES cells have been demonstrated to be pluripotent in vivo from their capacity to form teratocarcinomas (1 ,2 ) and germ-line chimeric mice (3 ), dependent on the environment into which the stem cells are introduced. When ES cells are introduced under the kidney capsule, in vivo differentiation is chaotic with the teratocarcinoma composed of a wide variety of different cell types. If, however, the stem cells are returned into a preimplantation mouse embryo, in vivo differentiation proceeds in a normal and organized manner, and the ES cells colonize the three primary cell lineages of the developing embryo: the primitive ectoderm, endoderm, and mesoderm. This leads to the formation of chimeric offspring composed of cells of two different genetic constitutions: the host embryonic cells and those derived from the ES cells. The ES cells are capable of contributing to every tissue in the fetus, including the primordial germ cells (3 ,4 ). Furthermore, the trophectodermal and primitive endodermal derivatives in the extraembryonic tissues of the conceptus may also be colonized by ES cells (5 ,6 ). Recent studies have shown that murine ES cells are capable of supporting complete fetal development, following the aggregation of stem cells with tetraploid-cleavage-stage embryos (7 ).