CyanoBase provides internet access to the complete genomic information of the cyanobacterium Synechocystis sp. strain PCC6803. CyanoBase contains annotations to each protein-coding gene, deduced from the entire nucleotide sequence of the genome, gene classification lists, keywords and similarity search engines. The present paper describes a recent extension of CyanoBase, named CyanoMutants. CyanoMutants is a repository database of mutant information on PCC6803. Each entry contains a dataset which describes a gene identifier, mutant information, and an address for correspondence. Two closely-linked databases, CyanoBase and CyanoMutants, connect information obtained from computational analysis to experimental analysis resulting in the clarification of the functions of hypothetical genes of the cyanobacterial genome. CyanoMutants can be accessed at http://www.kazusa.or. jp/cyano/mutants/