H2AX and cancer: As seen in HeLa cells, in both cancer cell types, which were almost completely lacking γ-H2AX staining before the treatment, we observed at the earliest time point (12 h) of HU treatment the appearance of γ-H2AX foci and progressively over time the nuclei of treated cells became pan-γ-H2AX stained (Figure 2A and Supplementary Figure S3A).