Cross-mapping of gene and protein identifiers between different databases is a tedious and time-consuming task. To overcome this, we developed CRONOS, a cross-reference server that contains entries from five mammalian organisms presented by major gene and protein information resources. Sequence similarity analysis of the mapped entries shows that the cross-references are highly accurate. In total, up to 18 different identifier types can be used for identification of cross-references. The

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... of the mapping could be improved substantially by exclusion of ambiguous gene and protein names which were manually validated. Organismspecific lists of ambiguous terms, which are valuable for a variety of bioinformatics applications like text mining are available for download. Availability: CRONOS is freely available to non-commercial users at