The manuscript from Langston and O'Donnell investigates the role of MCM10 in eukaryotic helicase function. They show convincingly that a stable complex forms between MCM10 and the CMG, and that the MCM10/CMG complex shows a dramatic improvement in helicase activity over the CMG alone. Although these are significant findings in their own right, they claim that MCM10's primary function is not to improve CMG action per se (as MCM10's effect on the CMG is eclipsed by that of the MTC complex), but to fundamentally alter the mechanism of translocation, isomerizing the CMG between a double-stranded to single-stranded encirclement mechanism for the purpose of bypassing blocks. All known hexameric DNA helicases translocate on one strand and exclude the other strand of a DNA fork. Langston, O'Donnell and colleagues previously observed that the eukaryotic replicative helicase, CMG, cannot bypass a roadblock on either the translocation strand (leading strand template) or the sterically excluded lagging strand. The structure of a fork-engaged CMG was used to explain this observation: duplex DNA is encircled by a B domain crown and the fork nexus is contacted by the C domain "OB hairpins", which guard the single-stranded DNA interacting ATPase ring pore. Given this mode of binding, a roadblock on either strand of the duplex DNA would therefore be impaired from physically reaching C domain hairpins. Langston and colleagues now reconstitute a CMG-Mcm10 complex and find that Mcm10 makes the CMG a more vigorous helicase, capable of bypassing a roadblock on the lagging strand. The authors suggest that Mcm10 induces an isomerization of the CMG that might release the duplex DNA portion of a fork from the B-domain embedding. This event would occur upon initiation of DNA replication and when the replisome needs to bypass a roadblock on the DNA.