Background: The bacteriophage-derived Cre–loxP recombination system operates efficiently in mammalian cells. This system is particularly useful in gene-targeting experiments in the mouse, and has already been used to generate ‘clean' deletions of target genes in the germ line, as well as to inactivate target genes in a conditional manner (based on regulated expression of the Cre recombinase). In principle, Cre–loxP-mediated recombination should also allow gene replacement, and thus the introduction of virtually any kind of mutation into the genome.

Results We used the Cre–loxP system, in mouse embryonic stem cells, to replace the mouse gene Cγ 1, which encodes the constant region of the heavy chain of IgG1 antibodies, with its human counterpart. The mutation was transmitted through the mouse germ line, and the resulting mutant mice were crossed with mice expressing κ light chains with a human, instead of a mouse, constant region. Mice homozygous for both mutations produce humanized, κ-chain-bearing IgG1 antibodies at the same level and efficiency as wild-type mice produce murine IgG1 antibodies. These animals should enable the ex vivo production of humanized, chimeric monoclonal antibodies specific for any antigen to which the mouse can respond.

Conclusion Cre–loxP-mediated gene replacement is a simple and efficient general method of targeted mutagenesis in the mouse.