Site-specific recombinase technology

Nearly every human gene has a counterpart in the mouse (regardless of the fact that a minor set of orthologues had to follow species specific selection routes). This made the mouse the major model for elucidating the ways in which our genetic material encodes information. In the late 1980s gene targeting in murine embryonic stem (ES-)cells enabled the transmission of mutations into the mouse germ line and emerged as a novel option to study the genetic basis of regulatory networks as they exist in the genome. Still, classical gene targeting proved to be limited in several ways as gene functions became irreversibly destroyed by the marker gene that had to be introduced for selecting recombinant ES cells. These early steps led to animals in which the mutation was present in all cells of the body from the beginning leading to complex phenotypes and/or early lethality. There was a clear need for methods to restrict these mutations to specific points in development and specific cell types. This dream became reality when groups in the USA were able to introduce bacteriophage and yeast-derived site-specific recombination (SSR-) systems into mammalian cells as well as into the mouse