In many cases, a gene "knockout" results in early embryonic lethality, which obscures the study of potential later functions. In other cases, the "knockout" does not show any phenotype due to the compensation of the gene deficiency by other family members. These limitations have called for further development of the powerful gene-targeting technology. One of the critical tools now being efficiently combined with gene-targeting is site-specific recombination. As the site-specific recombinase technology developed further in the mouse system, it became evident that this tool was going to have a significant impact on the power of mammalian genetics. The number of transgenic mouse lines expressing Cre recombinase with different specificities has steadily increased in the past 15 years and has now surpassed 500. Efficient utilization of this community-generated resource calls for a user-friendly database with all necessary information available about the properties of the Cre transgenic lines. The "CreXmice" database was created to meet these needs and has evolved over the past 4 years from flat file listings of transgenic lines into its current form, a professionally hosted SQL-driven web application. With hundreds of transgenic mouse lines, CreXmice is enriched by its presence on the World Wide Web allowing visitors the opportunity to search or contribute to the global effort by submitting the specific lines being developed by their laboratories.