DNA binding assays indicate that Mcm4/6/7 binds to those substrates containing single-stranded DNA regions regardless of the presence or absence of single-stranded 3′ or 5′ end. Unwinding of the duplex DNA depends on translocation of single-stranded DNA from 3′ to 5′ direction. This is most clearly shown by its helicase action on A-fork[5′] and 3′-extension but not on A-fork[3′], A-fork[3′,5′] nor 5′-extension. However, it does not necessarily require 3′ end of single-stranded DNA, since Mcm4/6/7 can displace annealing oligonucleotide on a circular single-stranded DNA. The ability of the mouse Mcm4/6/7 to unwind 3′-extension is shared by the archaeal Mcm helicase but not by Mcm4/6/7 from S.pombe and S.cerevisiae (14,26,27). The archaeal Mcm can unwind A-fork[3′] but eukaryotic Mcm4/6/7 cannot, since the former binds to double-stranded DNA but the latter does not (5,27). While yeast Mcm helicases can translocate on duplex DNA, such activity was not observed with mammalian Mcm4/6/7 (data not shown).