H2AX and esophageal squamous cell carcinoma: Moreover, both normoxia and hypoxia conditions presented γ-H2AX foci at 30 min, although ESCC cells in hypoxia (50 μM CoCl2 or 0.5-1% O2 levels) showed a notable decrease over the time (Fig. 1c), indicating lesser DNA damage and higher radioresistance.