Cre site-specific DNA recombinase system has been developed as a novel powerful tools for manipulating DNA both in vitro and in vivo. It has been used in transgenic mice to induce site-specific DNA recombination leading to gene expression or deletion/mutation not only in a tissue-specific or at a certain stage of development, by combining with inducible systems for controlling Cre expression or function, but also in temporally and spatially manner. These recombination-based strategies are likely to have a profound impact on study of gene function and the generation of animal models of human diseases.