Manipulating the genome by homologous recombination in embryonic stem cells.

The number of cloned genes has increased exponentially in the last decade. Often, the function of those genes is unknown. And, considering the enormous efforts to map and sequence the human and mouse genome, the gap between the availablity of structural information and functional assignments is likely to increase. The recent development of gene targeting, however, now enables us to study gene function in the living animaL Once a gene is isolated, a mutation can first be introduced into the cloned DNA and then, by homologous recombination, into the genome of mammalian cells. This procedure is generally referred to as gene targeting. If the cell in which gene targeting occurs is pluripotent [e.g. an oocyte or an embryonic stem (BS) cell], animals that bear this mutation in their germ cell lineage can be generated. Gene targeting probably has a large variety of applications in many different fields, including agricultural production and human gene therapy. In this review, however, I only evaluate gene targeting as a means to generate mice with specific mutations. I describe some technical aspects of gene targeting and then summarize the progress that has been made.