This domain is found at the N-terminus of the methylase subunit of Type I DNA methyltransferases. This domain family is found in bacteria and archaea, and is typically between 123 and 138 amino acids in length. The family is found in association with pfam02384. Mutations in this region of EcoKI methyltransferase abolish the normally strong preference of this system for methylating hemimethylated substrate. The structure of this domain has been shown to be all alpha-helical.