Two loxP sites are inserted on each side of an essential exon (2) of the gene of interest (i.e., gene X) (blue) by homologous recombination. These sites do not disrupt gene function. The loxP-containing mouse is crossed to a transgenic mouse carrying a cell-type-specific promoter controlling expression of the Cre recombinase, which induces recombination between loxP sites. This mouse is heterozygous for a constitutive gene X knockout. In the resulting loxP-Cre mouse, Cre protein is produced only in those cells in which the promoter is active, and in those cells recombination therefore occurs between the loxP sites, leading to deletion of exon 2. Since the other allele is a constitutive gene X knockout, deletion between the IoxP sites results in complete loss of function in all cells expressing Cre.