First, the endogenous gene to be targeted is modified by homologous recombination in murine embryonic stem (ES) cells so that LoxP sites flank a portion of the gene, and mice are generated from these recombinant ES cells to obtain flox (flanked by lox) mice.
Second, the heterozygous flox mice are bred with mice expressing a Cre transgene to generate double heterozygous mice. These double heterozygous mice are bred with single heterozygous flox mice to generate conditional knockout mice