While the human genome project has been completed, analysis of functions of each gene is still underway. Knockout and knock down of gene products offer useful tools to understand functions of a single gene in vivo. Production of knockout mice using homologous recombination in embryonic stem (ES) cells is a powerful and established strategy. However, it is laborious, time-consuming and expensive if expanding large scale. In mice, the gene trap is an alternative strategy to disrupt gene functions by random disruption of gene. The functions of a gene in vivo can be analyzed by production of mice from trapped ES clones. Large-scale gene trap projects have been started in some research centers of the world, and the International Gene Trap Consortium (IGTC) was established to strengthen interactions among centers involved. Moreover, the website of the IGTC has been constructed to integrate information of trap clones from each gene trap project. The database of the IGTC is expanding rapidly because of accumulation of information about gene trap clones from ongoing gene trap projects; approximately 135,000 trapped ES lines are registered in June, 2008. These clones are freely available to academic community. At moment, the IGTC cell lines have covered approximately 10,000 genes in the mouse genome database. Therefore, it is recommended to check the IGTC database before starting knockout experiment, even when annotations of genes are not available. In this review, we introduce principle and short history of gene trap, and then use of the IGTC database is described to obtain trapped ES clones for the experiments.