With the completion of the human and mouse genome sequences, there has been increased interest in developing tools for genome-wide mutagenesis in the mouse, with the goal of analyzing genome function in the context of the intact organism. Gene trapping is a high-throughput approach that can be used to introduce insertional mutations across the genome in mouse embryonic stem (ES) cells. Gene trap vectors simultaneously mutate and report the expression of the endogenous gene at the site of insertion and provide a DNA tag for the rapid identification of the disrupted gene. The generation of mutant mice from ES cell lines carrying gene trap insertions could be applied to large-scale functional analysis of mammalian genes. The International Gene Trap Consortium (IGTC) consists of laboratories around the world working together to generate a public library of mutated murine ES cell lines. Such cell lines can be obtained on a non-collaborative basis by scientists interested in generating reporter-tagged, loss-of-function mutations in mice. In addition to loss of function, newer gene trap vectors offer a variety of post-insertional modification strategies to allow for the generation of other experimental alleles. The cooperative goal of the IGTC is to generate an international resource representing all or most genes in the mouse genome, and to provide the bioinformatics and logistical support to make the resource valuable and available to scientists.