Segmental genomic replacement in embryonic stem cells by double lox targeting.

We have applied Cre-mediated double lox recombination to embryonic stem (ES) cells to facilitate repeated knock-ins at a target locus, thus helping to assure correct temporal and spatial transgene expression in mice. Using homologous recombination, we inserted a double lox cassette a few nucleo-tides before the authentic ATG start of MHL-1, the gene coding for the major subunit of the asialoglycoprotein receptor. The cassette carries a marker gene bounded by heterospecific lox sites that cannot recombine with each other, but which can undergo recombination with like sites on an incoming double lox targeting vector. Cre-mediated replacement of the lox-delimited genomic segment at MHL-1 with targeting DNA occurs at a frequency three times that of random DNA integration and is sufficiently robust that correctly targeted ES colonies can be identified by PCR screening without relying on any drug selection procedure. Moreover, double lox recombination at the MHL-1 locus is not dependent on the presence or absence of a transcriptionally active promoter at the genomic target. The strategy and vectors described here are generally applicable to designing double lox targeted knock-ins at any locus in ES cells and should prove useful in more precise molecular engineering of the mouse genome.