REBASE is a comprehensive and extensively curated database of information about the components of restriction-modification (RM) systems. It is fully referenced and provides information about the recognition and cleavage sites for both restriction enzymes and DNA methyltransferases together with their commercial availability, methylation sensitivity, crystal and sequence data. All completely sequenced genomes and select shotgun sequences are analyzed for RM system components. When PacBio sequence data is available, the recognition sequences of many DNA methyltransferases (MTases) can be determined. This has led to an explosive growth in the number of well-characterized MTases in REBASE. The contents of REBASE may be browsed from the web rebase.neb.com and selected compilations can be downloaded by FTP (ftp.neb.com). Monthly updates are also available via email.