Twenty years of programmed modifications of the mouse genome: a revolutionin the genetic approach of mammalian biology

The ability to introduce genetic modifications in the germline of complex o rganisms has been a long standing goal of those who study developmental bio logy. In this regard, the mouse, a favorite model for the study of mammals, is unique: indeed not only is it Possible since the late seventies, to add genes to the mouse genome like in several other complex organisms but also to perform gene replacement and modification, This has been made possible via two technological breakthroughs: (1) the isolation and culture of embry onic stem cells (ES), which have the unique ability to colonize all the tis sues of an host embryo including its germline; (2) the development of metho ds allowing homologous recombination between an incoming DNA and its cognat e chromosomal sequence (gene much less than targeting much greater than). A s a result, it has became Possible to create mice bearing null mutations in any cloned gene (knock-out mice). Such a possibility has revolutionized th e genetic approach of almost all aspects of the biology of the mouse. In re cent years, the scope of gene targetting has been widened even more, due to the refinement of the knock-out technology: other types of genetic modific ations may now be created, including subtle mutations (point mutations, mic ro deletions or insertions, etc.) and chromosomal rearrangements such as la rge deletions, duplications and translocations. Finally, methods have been devised which permit the creation of conditional mutations, allowing the st udy of gene function throughout the life of an animal, when gene inactivati on entails embryonic letality. in this paper, we present an overview of the methods and scenarios used for the programmed modification of mouse genome , and we underline their enormous interest for the study of mammalian biolo gy.