Ability to rapidly bypass blocks on the non-tracking strand is inherent in all replicative hexameric helicases examined with the exception of eukaryotic CMG. One possible reason that CMG evolved a requirement for another protein to bypass a lagging strand block is for regulation of fork initiation and progression. It is documented from several laboratories that the head-to-head (N-to-N) double hexamer of Mcm2-7 at an origin matures to form two CMGs on dsDNA without unwinding DNA (Heller et al., 2011; Kanke et al., 2012; van Deursen et al., 2012; Watase et al., 2012; Yeeles et al., 2015). Mcm10 is required at the last step of origin initiation to ‘activate’ CMG to unwind DNA as detected by RPA binding to the ssDNA products. These observations can be explained by the findings of this report. Specifically, CMG is known to track with the N-tier ahead of the C-tier, and therefore head-to-head CMGs at an origin are directed inward toward one another. Hence, each CMG blocks the other CMG at an origin. However, if Mcm10 activates the CMGs to locally unwind DNA, as observed by head-to-head SV40 T-antigen hexamers that produce ssDNA ‘rabbit ears’ at the viral origin (Wessel et al., 1992), the two CMGs could isomerize to encircle opposite single-strands and pass one another to form bidirectional forks (see Figure 8B in [Georgescu et al., 2017]). This same action may also promote termination, when two forks meet, enabling the CMG based replisomes to easily pass one another.