The analysis of phenotypes caused by null and mutant alleles is a very powerful means to understand gene function in vivo. Historically, this experimental approach has been widely and successfully used in invertebrate models. Now, thanks to the gene-targeting technology in ES cells, the genome of a mammalian organism such as the mouse can be artificially modified by precise alterations. The system exploits the ability of ES cells to be cultured and manipulated in vitro without losing their totipotency (1,2). Mutations in specific genes can be achieved by in vitro selection of ES cell clones in which the locus of interest has been targeted by homologous recombination (3,4). The peculiar property of being totipotent allows ES cells, once injected in the cavity of a blastocyst, to contribute to the formation of all cell types of a chimeric embryo. Whenever a chimeric mouse possesses ES-derived germ cells, the mutation can be propagated to its offspring. Heterozygous mice are then mated to generate the homozygous offspring needed for phenotypic analysis.