Introduction Sequencing of whole genomes has provided new perspectives into the blueprints of diverse organisms, including the genome of the mouse (Waterston et al., 2002). Although the complete sequence is now available, the estimation of total gene number encoded by the mouse genome is ranging approximately from 25,000 to 50,000 (Okazaki et al., 2002). This uncertainty about the functional units within the genome highlights the importance of a detailed analysis of the encoded genes. A significant step toward a better understanding of the genome has been the development of large-scale gene expression analysis tools utilizing DNA microarrays (Bono et al., 2003). This technology allows the generation of gene expression profiles that can give important clues for the interpretation of biological processes. However, the obtained data do not directly address the function of individual genes. Rather, they present a snapshot of global gene expression changes. While this is a very useful parameter for understanding the genome, it is not very useful for studying detailed phenotypic changes after gene ablation. About 15 years ago gene function analysis became available in the mouse through the development of gene knock-out technology (Capecchi, 1989). In this approach genes are targeted in embryonic stem (ES) cells through homologous recombination. The manipulated ES cells are subsequently injected into blastocysts, and chimeric offspring are checked for germline transmission. Successful germline transmission allows the production of animals deficient in the gene of interest. Careful phenotypic analyses of these animals can then disclose the function(s) of the knocked-out gene.