EXPERIMENTAL FIGURE 6-39 The loxP-Cre recombination system can knock out genes in specific cell types. A loxP site (purple) is inserted on each side of an essential exon (exon 2) of the target gene (gene X; blue) by homologous recombination, producing a loxP mouse. Since the loxP sites are in introns, they do not disrupt the function of X. A Cre mouse carries one gene X knockout allele and an introduced cre gene (orange) from bacteriophage P1 linked to a cell-type-specific promoter (yellow). The cre gene is incorporated into the mouse genome by nonhomologous recombination and does not affect the function of other genes. In the loxP-Cre mice that result from crossing these two types of mice, Cre protein is produced only in those cells in which the promoter is active. Thus these are the only cells in which recombination between the loxP sites catalyzed by Cre occurs, leading to deletion of exon 2. Since the other allele is a constitutive gene X knockout, deletion between the loxP sites results in complete loss of function of gene X in all cells expressing Cre. By using different promoters, researchers can study the effects of knocking out gene X in various types of cells.