Novel genetically engineered retroviral vectors and targeting plasmids are described that enable the site-specific targeting of exogenous DNA into the genomes of cultured animal cells. The protocol involves the transduction of competent cells by a chimeric retroviral vector containing a transcription unit composed of two linked cassettes an upstream marker gene under the control of the viral 5 LTR; and a downstream reporter trap containing a strong promoter 5 to a 48 bp yeast FRT element. When cells containing such integrated units are co-transfected with a plasmid encoding yeast FLP recombinase and a promoterless targeting plasmid containing a reporter cDNA tract 3 to an homologous FRT element, the targeting plasmid recombines at the chromosomally preconfigured FRT site and a new hemizygous function is introduced into the downstream cassette. These reagents provide a new portable system for site-specific targeting of chemically modified genes into uniform and unique sites in genomically integrated transcription units.