ES cells are early embryo-derived multipotential cells that can be genetically manipulated in tissue culture. Thus genetic modification can be introduced into the gene pool by injecting transfected ES cells into blastocysts.The unique advantage that ES cells offer is thatthey can be screened in culture for rare geneticevents prior to germline transmission and the desired genetic mutant can be produced.Recently, genes which are involved in mousedevelopment can be identified using gene trap vectors in combination with ES cells. This new approach is very efficient for studying gene expression pattern during embryogenesis.