Cell biological and genetic studies have shown that replication forks move about 2-fold slower in cells that lack Mrc1 (Hodgson et al., 2007; Petermann et al., 2008; Szyjka et al., 2005; Tourrière et al., 2005) and these observations have been recapitulated in vitro in a origin plasmid replication system (Yeeles et al., 2017). The in vitro results demonstrated that Mrc1 stimulation of fork speed was most efficient in the presence of a Tof1-Csm3 heterodimeric complex. The stimulation by Mrc1 ± Tof1-Csm3 in the origin plasmid based assay did not require Ctf4, FACT, TopI or lagging strand Pol δ extension (Yeeles et al., 2017). Mcm10 is essential for initiation in the plasmid assay system and this requirement presents difficulty in determining whether Mcm10 also enhanced the rate of replication fork progression when Mrc1-Tof1-Csm3 was present (Yeeles et al., 2017). In our experiments, CMG is added as a purified complex enabling a direct test of whether Mcm10 stimulates fork speed in the presence of these additional factors. Hence we expressed and purified a Mrc1-Tof1-Csm3 heterotrimer, referred to herein as MTC complex, and tested its effect on the leading strand replisome with and without Mcm10 (Figure 4C). The autoradiogram shows that in the presence of the MTC complex, full length 2.8 kb product can be observed after 3 min (Figure 4C, lane 6), indicating a rate of 20 ntds/s (933 ntds/min), close to the 1.5–1.9 kb/min rate of replisomes in vivo (Conti et al., 2007; Hodgson et al., 2007; Sekedat et al., 2010). Furthermore, the MTC complex clears up the many immature products formed in the absence of MTC, suggesting that replication forks that stop or pause are brought to full length by the MTC complex. Addition of Mcm10 to leading strand replisome reactions containing the MTC complex does not result in a faster replication fork rate under the conditions used and may have even slightly slowed the replisome (Figure 4C, lanes 11–15). Thus, the improved rate effect of Mcm10 on the core leading strand replisome observed in Figure 4B is eclipsed by the rate enhancement provided by the MTC complex.