Restriction modification systems are the most abundant antiphage systems. They already have their own [Wikipedia page](https://en.wikipedia.org/wiki/Restriction_modification_system)
## Molecular Mechanisms
Several reviews detail the molecular mechanisms of restriction modification systems. For example in :ref{doi=10.1016/j.mib.2005.06.003}:
Several reviews detail the molecular mechanisms of restriction modification systems. For example in :ref{doi=10.1016/j.mib.2005.06.003}:
"Bacterial restriction-modification (R-M) systems function as prokaryotic immune systems that attack foreign DNA entering the cell :ref{doi=10.1128/jb.65.2.113-121.1953}. Typically, R-M systems have enzymes responsible for two opposing activities: a restriction endonuclease (REase) that recognizes a specific DNA sequence for cleavage and a cognate methyltransferase (MTase) that confers protection from cleavage by methylation of adenine or cytosine bases within the same recognition sequence. REases recognize ‘non-self’ DNA (Figure 1), such as that of phage and plasmids, by its lack of characteristic modification within specific recognition sites :ref{doi=10.1093/nar/29.18.3705}. Foreign DNA is then inactivated by endonucleolytic cleavage. Generally, methylation of a specific cytosine or adenine within the recognition sequence confers protection from restriction. Host DNA is normally methylated by the MTase following replication, whereas invading non-self DNA is not."
