The specific requirement of single-stranded thymine residues for activation of Mcm helicase prompted us to examine whether they are required also for processive unwinding of duplex DNA. Our results indicated that increase of GC pair in the duplex segment significantly inhibited the Mcm helicase activity. Duplex DNA composed only of GC pairs (10 repeats of CGG) on a Y-fork was not displaced at all, while the same template was readily displaced by SV40 T-antigen (Figure 8B). Mcm helicase was inhibited by the presence of GC-rich duplex segment also on a circular single-stranded partial heteroduplex substrate. However, on this substrate, Mcm4/6/7 was able to displace DNA past the GC-rich region, albeit to a limited extent, when it was added at a high concentration. On the partial heteroduplex template, Mcm is loaded onto the circular single-stranded DNA of 6.4 kb, while it is loaded onto the 50 nt long 3′-tail DNA on the Y-fork. Thus, the difference of helicase actions may reflect the efficiency of Mcm loading. Alternatively, the presence of ‘random’ sequence at the initially unwound duplex segment in the former template may engage the Mcm helicase in a more active conformation which can displace the GC-rich duplex segment.