Gene targeting--homologous recombination of DNA sequences residing in the chromosome with newly introduced DNA sequences--in mouse embryo-derived stem cells promises to provide a means to generate mice of any desired genotype. We describe a positive nd negative selection procedure that enriches 2,000-fold for those cells that contain a targeted mutation. The procedure was applied to the isolation of hprt- and int-2- mutants, but it should be applicable to any gene.