We next examined the effect of GC-rich segments on a partial heteroduplex DNA on a single-stranded circular DNA. We have constructed two sets of 5′-tailed partial heteroduplex DNA substrates containing duplex regions of variable lengths; one on M13mp18 vector and the other on M13mp19 containing a 66 nt long G-rich segment downstream of the hybridizing oligonucleotide. We found that the Mcm4/6/7 helicase can displace duplex DNA up to 350 nt long on its own on the former substrate (Figure 8D, left), consistent with previous results (13). In contrast, displacement was inhibited over the GC-rich segments on the latter substrate (Figure 8D, right panel), although displacement up to 250 nt in length, albeit at a reduced level, was observed at a high concentration of Mcm4/6/7 complex. This result is consistent with that on Y-fork and indicates that GC-rich duplex segment is inhibitory for unwinding by the Mcm helicase. These results may indicate that the presence of thymine–adenine pairs with a certain frequency in the duplex region may facilitate continuous unwinding by Mcm4/6/7. Alternatively, Mcm4/6/7 is not efficient enough to displace thermodynamically stable GC-base-paired segment.