(Related to Fig. 4 ): Addition of proteinase K increases the percentages of broken forks detected by spreading. Blue–green–blue versus blue–green forks scored after the addition of proteinase K to the spreading protocol. DNA spreading assay performed by the Chaudhuri laboratory as depicted in Fig. 3 A. Data are presented as a percentage of the total scored pulse-labeled tracts. At least 100 tracts were scored for each sample. Numbers indicate the mean tracts %. Statistics: mean ± SEM. Number of biological repeats N = 3.