The CreZOO (http://www.crezoo.org/) is the European virtual repository of Cre and other targeted conditional driver strains. These mice serve as tools for researchers to selectively ‘switch off’ gene expression in mouse models to examine gene function and disease pathology. CreZOO aims to capture and disseminate extant and new information on these Cre driver strains, such as genetic background and availability information, and details pertaining promoter, allele, inducibility and expression patterns, which are also presented. All transgenic strains carry detailed information according to MGI's official nomenclature, whereas their availability [e.g. live mice, cryopreserved embryos, sperm and embryonic stem (ES) cells] is clearly indicated with links to European and International databases and repositories (EMMA, MGI/IMSR, MMRRC, etc) and laboratories where the particular mouse strain is available together with the respective IDs. Each promoter/gene includes IDs and direct links to MGI, Entrez Gene, Ensembl, OMIM and RGD databases depending on their species origin, whereas allele information is presented with MGI IDs and active hyperlinks to redirect the user to the respective page in a new tab. The tissue/cell (special) and developmental (temporal) specificity expression patterns are clearly presented, whereas handling and genotyping details (in the form of documents or hyperlinks) together with all relevant publications are clearly presented with PMID(s) and direct PubMed links. CreZOO's design offers a user-friendly query interface and provides instant access to the list of conditional driver strains, promoters and inducibility details. Database access is free of charge and there are no registration requirements for data querying. CreZOO is being developed in the context of the CREATE consortium (http://www.creline.org/), a core of major European and international mouse database holders and research groups involved in conditional mutagenesis.