ABSTRACT
The mouse, whose genome is similar to that of humans both in size and gene number, is considered as an excellent animal model for defining the functions of human genes as it exhibits reproductive, developmental, physiological, and pathological similarities to man. The generation of targeted mutations in the mouse genome is expected to play a crucial role in the elucidation of the physiological functions of mammalian genes and in the characterization of genetic factors involved in human pathologies (1). The introduction of targeted mutations in the germ line of the mouse has markedly advanced our understanding of the roles played by a number of gene products during development and adult life. However, this approach has some inherent limitations, as the lack of a protein that serves essential functions during development may result in early lethality, possibly in utero, thus precluding analysis of its possible functions at subsequent stages. Furthermore, numerous genes exert multiple functions in distinct cell types during ontogeny and in postnatal life (pleiotropy). Germline ablation of such genes may result in complex phenotypes, in which it may be impossible to distinguish events
that involve cell-autonomous effects from those more complex that involve cell-nonautonomous effects. The outcomes of a germ line mutation may also be compensated during development, thus preventing the appearance of an abnormal phenotype at subsequent stages. In addition, in the case of closely related genes belonging to a multi-gene family, the mutation of a given member of the family may not result in an abnormal phenotype, due to functional redundancies among the family members (possibly artefactually generated by the mutation), thus precluding the identification of the function of individual members of the family and requiring the generation of compound mutants to reveal the function(s) played by the gene family. Defining the role of individual family members may become even more difficult when the family is involved in highly pleiotropic signalling pathways, such as those of nuclear receptors, for example, retinoid receptors (2-5). Other potential effects confounding conventional germline knockouts include the risk of impaired fertility and generalized, systemic disorders (6,7). In many instances, these limitations in targeted germ line mutagenesis prevent the determination of the function of a given gene product in a defined subset of tissue/ cell-type at any given time during the animal’s life. Moreover, germline knockouts prevent engineering of mouse models for human diseases that are caused by somatic mutations, particularly when these diseases result, as in most cancers, from a combination of somatic mutations (8).
