Many bacteria are able to resist infection by bacteriophages (viruses that are specific for bacteria) by virtue of a restriction–modification system. The bac- terium modifies certain nucleotides in specific sequences of its own DNA by adding a methyl (¬CH 3 ) group in a reaction catalyzed by a modification methylase. A restriction endonuclease, which recognizes the same nucleotide sequence as does the methylase, cleaves any DNA that has not been modified on at least one of its two strands. (An endonuclease cleaves a nucleic acid within the polynucleotide strand; an exonuclease cleaves a nucleic acid by re- moving one of its terminal residues.) This system destroys foreign (phage) DNA containing a recognition site that has not been modified by methyla- tion. The host DNA is always at least half methylated, because although the daughter strand is not methylated until shortly after it is synthesized, the parental strand to which it is paired is already modified (and thus protects both strands of the DNA from cleavage by the restriction enzyme).