Members of this family are predominantly found in prokaryotic restriction endonuclease BglII, and adopt a structure consisting of an alpha/beta core containing a six-stranded beta-sheet surrounded by five alpha-helices, two of which are involved in homodimerisation of the endonuclease. They recognise the double-stranded DNA sequence AGATCT and cleave after A-1, resulting in specific double-stranded fragments with terminal 5'-phosphates [1].

1: Understanding the immutability of restriction enzymes: crystal structure of BglII and its DNA substrate at 1.5 A resolution. Lukacs CM, Kucera R, Schildkraut I, Aggarwal AK; Nat Struct Biol. 2000;7:134-140. PMID:10655616