[Preparation and application of knock-out mice]

Mouse embryonic stem cells can be cultured in vitro and still retain their pluripotency. When reintroduced into mouse embryos, these cells can contribute to the germ line of the resultant chimeras. Homologous recombination between DNA sequences residing in the chromosome of embryonic stem cells and newly introduced DNA sequences allows the transfer of any modification of the cloned gene into the genome of a living mouse. This article discusses the current status of knock-out mice preparation, with emphasis on unsettled steps, during the knock-out procedure and introduces some of the newly developed gene targeting techniques by which more ideal animals can be prepared.