With gene targeting, instead of a foreign transgene being introduced, an endogenous gene in the mouse is modified. Initially, the modification is made in specialized cells termed embryonic stem (ES) cells. ES cell lines are derived from early-stage mouse embryos and can be maintained indefinitely in an undifferentiated state in vitro yet retain the capacity, when injected back into an early-stage mouse embryo, to mix with the endogenous cells of the embryo and contribute to all tissues of the developing mouse, including the germ line. The gene of interest is modified in ES cells by the introduction of a targeting vector that consists of a modified version of the endogenous gene. In ES cells, the targeting vector recombines with the homologous endogenous gene and thereby introduces the genetic modification. Gene targeted ES cells are then injected into wild type blastocyst-stage mouse embryos with the chimeric mice that result being mixtures of the modified ES cells and wild type blastocyst cells. The successful integration of the ES cells into the germ line permits the genetic modification to be propagated as part of the mouse genome, and this creates stable transgenic lines.
