The use of site-specific recombinase systems and gene targeting techniques as useful tools for activating and inactivating specific genes of mice in a 'conditional' manner in spatially and temporally restricted patterns is reviewed. These methodologies are particularly useful for elucidating a complete picture of gene function in which: (1) the conventional knockout leads to an early lethal phenotype preventing the study of its later roles and/or (2) the phenotype affects multiple tissues, preventing the detailed study of its function in a particular cell lineage.