romosome maintenance (Mcm) helicase belongs, are known to assemble a ring-like structure (3–5). Whereas most of DNA helicases of this group, including simian virus 40 large T-antigen (6), Escherichia coli DnaB (7) and Archaeal Mcm (4,8,9), are composed of a single subunit, the active eukaryotic Mcm helica