In prokaryotic DNA the major methylated bases are N⁶-methyladenine (mA) and to a lesser extent N⁴-methylcytosine. Methylation in bacteria occurs at specific sites. In E. coli, methylation of A residues in the sequence 5´-GATC-3´ is involved in mismatch error correction, and it plays a role in controlling initiation of DNA replication. Methylation at other sites protects DNA against cleavage by restriction endonucleases (described here).