REBASE is a comprehensive database of information about restriction enzymes, DNA methyltransferases and related proteins involved in the biological process of restriction modification. It contains fully referenced information about recognition and cleavage sites, isoschizomers, neoschizomers, commercial availability, methylation sensitivity, crystal and sequence data. Experimentally characterized homing endonucleases are also included. All newly sequenced genomes are analyzed for the presence of putative restriction systems and this data is included within REBASE.