Molecular techniques now allow the design of precise genetic modifications in the mouse. Not only can defined nucleotide changes be engineered into the genome of the mouse, but genetic switches can be designed to target expression or ablation of any gene (for which basic molecular information is available) to any tissue at any defined time. These strategies promise to contribute substantially to an increased understanding of individual gene function in development and pathogenesis. A powerful tool, both for the design of such genetic switches and for speeding the creation of gene-modified animals, is the Cre site-specific DNA recombinase of bacteriophage P1. Precise DNA rearrangements and genetic switches can be efficiently generated in a straightforward manner using Cre recombinase. In conjunction with inducible systems for controlling Cre expression and function, these recombination-based strategies are likely to have a profound impact on developmental biology and the generation of useful animal models of human disease.