Figure 3.

Schematic of DNA fiber technique. Cells are sequentially pulsed with the nucleotide analogs CldU and IdU and then treated with a replication stress inducing agent such as hydroxyurea. Cells are lysed, the DNA fibers are linearly arrayed on cover slips and stained with fluorescently tagged antibodies, and the fibers are visualized and counted. The ratio of the green first track to the red second track is calculated for each fiber. Tumor cells with stable forks that are more likely resistant to DDR inhibitors (DDRi) have a ratio of one while tumor cells with unstable forks have a ratio less than one and are more likely sensitive to DDRi.