MOTIVATION: Due to the development of high-throughput technologies such as microarrays, it has become possible to determine genome-wide expression changes in a single experiment. Although much attention has been paid to identify differentially expressed genes, the functions of tens of thousands of genes in different species still remain unknown. RESULTS: C-It is a knowledge database that has its focus on 'uncharacterized genes'. C-It contains expression profiles of various tissues from human, mouse, rat, chicken and zebrafish. By applying our previously introduced algorithm DGSA (Database-Dependent Gene Selection and Analysis), it is possible to screen for uncharacterized, tissue-enriched genes in the species mentioned above. C-It is designed to include further expression studies, which might provide more comprehensive coverage of gene expression patterns and tissue-enriched splicing isoforms. We propose that C-It will be an excellent starting point to study uncharacterized genes. AVAILABILITY: C-It is freely available online without registration at http://C-It.mpi-bn.mpg.dehttp://C-It.mpi-bn.mpg.de.