Conventional gene targeting has been very useful in the study of gene function and regulation in mice. However, the methodologies involved have several limitations. First, mutations that cause embryonic lethality largely preclude studies of gene function at a later stage in development. Second, conditional and/or tissue-specific alterations of gene expression cannot be achieved using these methods. In addition, classical gene targeting can be difficult and time consuming. Strategies that make use of site-specific recombinases such as Cre and/or Flp have been developed in recent years to overcome these limitations. These new techniques include global and conditional knockouts, recombinase-mediated DNA insertion (RMDI), and recombinase-mediated cassette exchange (RMCE). Together, they have tremendously increased the number and variety of genetic manipulations that can be achieved.