Lane 10 of the SDS-PAGE of Figure 2—figure supplement 1 is shown to the left. The densitometry scan of the SDS gel lane 10 (right) was analyzed using ImageJ and indicates 2CMGE−1Ctf43−1Pol α-primase. The area of the Cdc45 peak was used as a proxy for CMG stoichiometry, and the area of the Cdc45 peak divided by the Cdc45 mw was assigned a value 2.0 because molar areas of Pol1, Pol12, and Ctf43 were about half the molar value of Cdc45. The amount of Ctf43, which overlaps Mcm4, was determined in two steps. First, the area of the Mcm2-7 region was divided by the molecular weight of Mcm2-7. The difference in area was deduced to belong to Ctf43, and calculated based on the molecular weight of a Ctf4 trimer. Calculated values are shown under the peaks. The stoichiometry approximates to 2 Cdc45 (and thus 2 Mcm2-7), 1 Ctf43, 1 Pol α, and 2 Pol ε.