Conditional Gene Knockout Using Cre Recombinase

The directed introduction of null mutations into defined genes has proven invaluable in elucidating gene function in a variety of experimental organisms. In the last decade or so this approach has been extended to mice ( 1 ) by the combined use of homologous recombination in murine embryonic stem (ES) cells to precisely target a mutation to a desired gene and subsequent derivation of mice carrying the targeted gene alteration from the genetically manipulated ES cells (e.g., by injection of gene- modified ES cells into blastocysts with subsequent germline transmission). In most instances null, or knockout (KO), mutations have been generated in mice by either simple insertion of a neo selectable marker in the target gene or neo insertion coupled with deletion of a critical region of the target gene. Targeted null mutations in a gene of interest, however, can lead to embryonic lethality in mice, thus obscuring the particular role of that gene in a target tissue or in the adult.