In vivo, the two processes of DNA restriction and DNA modification are intrinsically linked and can be viewed as a protective measure for the bacteria against invasion by bacteriophages, or other sources of ‘foreign’ DNA. Restriction provides the bacterium with the protection against incoming ‘foreign’ DNA and is due to the presence of a restriction endonuclease, which binds to the DNA at specific target sites and cuts the DNA.  However, to prevent this endonuclease from cleaving the bacteria’s own DNA there is an associated modification system.  Modification involves a DNA methyltransferase, which recognises the same DNA sequence as the associated endonuclease, but adds methyl groups within this sequence.  The number of methyl groups added – one or two – would depend upon the existing methylation status at the site.  These methyl groups prevent DNA cleavage by the endonuclease.  In fact, the endonucleases