REBASE is a comprehensive database that provides information on restriction enzymes, DNA methyltransferases, and related proteins involved in the biological process of restriction-modification (R–M). It includes fully referenced details about recognition and cleavage sites, isoschizomers, neoschizomers, commercial availability, methylation sensitivity, crystal and sequence data. The database also features experimentally characterized homing endonucleases and putative R–M systems from sequence databases. Comprehensive descriptions of the R–M content in all fully sequenced genomes are available, including summary schematics. REBASE can be browsed online and selected compilations can be downloaded via FTP. Monthly updates are available via email. As of the latest update, REBASE contains 4990 restriction enzyme genes and 8080 DNA methyltransferase genes, with 3511 R and 5497 M genes from 1050 completely sequenced bacterial and archaeal genomes. The database includes links to other major databases such as GenBank, PubMed, UniProt, PDB, and Pfam. REBASE is a valuable resource for annotating bacterial and archaeal genomes, offering tools for sequence analysis and custom analyses of unpublished genome sequence data.REBASE is a comprehensive database that provides information on restriction enzymes, DNA methyltransferases, and related proteins involved in the biological process of restriction-modification (R–M). It includes fully referenced details about recognition and cleavage sites, isoschizomers, neoschizomers, commercial availability, methylation sensitivity, crystal and sequence data. The database also features experimentally characterized homing endonucleases and putative R–M systems from sequence databases. Comprehensive descriptions of the R–M content in all fully sequenced genomes are available, including summary schematics. REBASE can be browsed online and selected compilations can be downloaded via FTP. Monthly updates are available via email. As of the latest update, REBASE contains 4990 restriction enzyme genes and 8080 DNA methyltransferase genes, with 3511 R and 5497 M genes from 1050 completely sequenced bacterial and archaeal genomes. The database includes links to other major databases such as GenBank, PubMed, UniProt, PDB, and Pfam. REBASE is a valuable resource for annotating bacterial and archaeal genomes, offering tools for sequence analysis and custom analyses of unpublished genome sequence data.