Advances in the use of embryonic stem cell technology.

Embryonic stem cells have become versatile genetic vehicles. Two-step recombination protocols have been used to modify endogenous mouse genes, and yeast artificial chromosomes containing human genes have been transmitted into the mouse germline. Recently, it has become possible to evaluate homozygous deficiencies in specific developmental compartments either through the use of chimeras or by activating recombination in vivo using potent recombinases.