Several databases exist for restriction sites and enzymes, of which the largest noncommercial database is REBASE. Recently, it has been shown that statistically significant nullomers (i.e. short absent motifs which are highly expected to exist) in virus genomes are restriction sites indicating that viruses have probably got rid of these motifs to facilitate invasion of bacterial hosts. [https://www.nullomers.org/ Nullomers Database] contains a comprehensive catalogue of minimal absent motifs many of which might potentially be not-yet-known restriction motifs.