Gene targeting in embryonic stem cells: the new physiology and metabolism.

The development of transgenic technology, whereby genes (or mutations) can be stably introduced into the germline of experimental mammals, now allows investigators to create mice of virtually any genotype and to assess the consequences of these mutations in the context of a developing and intact mammal. In contrast to traditional "gain-of-function" mutations, typically created by microinjection of the gene of interest into the one-celled zygote, gene targeting via homologous recombination in pluripotential embryonic stem cells allows one to modify precisely the gene of interest. The purpose of this review is to introduce the reader to the history of development of embryonic stem cell technology, the current methods employed to create "knock-out" mice, and the application of these methods to solve problems in biology. While the technology promises to provide enormous insight into mammalian development genetics, our desire is that this review will stimulate the application of gene targeting in embryonic stem cells to begin to unravel problems in complex regulatory pathways, specifically intermediary metabolism and physiology.