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 these data are included within the REBASE. The contents or REBASE may be browsed from the web (http://rebase.neb.com/rebase/rebase.ftp.html) and selected compilations can be downloaded by ftp (ftp.neb.com). Additionally, monthly updates can be requested via email.