Decreased repair of DNA double strand breaks (DNA-DSBs) under continual hypoxia. A, Despite a decrease in the initial number of induced and sensed DSBs measured by γ-H2AX foci at 30 minutes following 2 Gy, hypoxic (0.2% O2) G0/G1 synchronized human fibroblasts have an increased number of residual γ-H2AX foci at 24 hours. The asterisk represents a significant difference (*P < 0.05) between oxic control (solid) and hypoxic treatment (dashed). Plot is adapted from data published in Kumareswaran et al. . B, Two dimensional (top panels) and three dimensional (bottom panel) confocal images of G0/G1 fibroblasts with increased number of residual γ-H2AX foci under continual hypoxia at 24 hours following 2 Gy of irradiation. Scale bar = 10 μm.