Constitutes a resource dedicated to in-depth analysis of bacterial and archaeal gene fusion events. FusionDB reduces the number of false positives, at the expense of a potentially similarly high number of false negatives. To recover from this drawback, gene fusion events between genes from different genomes that belong to the same cluster of orthologous groups (COG). The extension to COG fusion events also provides information on general gene fusion tendencies in a whole bacterial genomic context. FusionDB may be searched by gene name, gene annotation, gene function, COG identifier or simply by entering an amino acid sequence. Output in full-page mode contains visualization of the different alignments that were used for scoring, and in the case of gene pairs that both belong to a COG a special COG-analysis page is provided.