A number of studies have demonstrated that nitric oxide enhances radiosensitivity of anoxic and hypoxic cells in vitro and in vivo, and some evidence points to a role for DNA damage and repair in this phenomenon. We have recently observed that nitric oxide enhances the formation of DNA single- and double-strand breaks following ionising irradiation, measured by the alkaline comet assay and immunofluorescence microscopy for γ-H2AXγ-H2AX.