Laboratory and clinical data suggest that a new paradigm using LDFRT as a chemopotentiator may allow full-dose drug therapy with improved efficacy without adding to the toxicity of the systemic treatment. This chemopotentiating effect is possible through a phenomenon known as hyper-radiation sensitivity (HRS) by which there is more effective tumor cell killing than would be predicted when using doses per fraction below 1 Gy. This is followed by a change in slope of the survival response with increasing doses per fraction, indicating increased radioresistance (IRR). This HRS phenomenon was first described by Joiner and colleagues in the Gray Laboratory in 1986 and has since been well described by a number of other laboratories. It also has been documented in the clinical setting; in a study by Harney et al., patients with paired cutaneous metastases from sarcoma and melanoma had longer time to tumor regrowth after LDFRT than with conventional radiation. In vitro studies have established a link between HRS/IRR and evasion of the early G2/M cell cycle checkpoint. Exaggerated HRS/IRR responses were found for enriched populations of G2 phase cells in one study, indicating that the mechanism likely involved events in the G2 phase of the cell cycle. Two G2 checkpoints have been described, and the more recently discovered "early" checkpoint is rapidly activated after radiation exposure. It is believed to prevent cell cycle progression through G2 of cells with unrepaired radiation-induced DNA damage. The signaling cascade regulating the early G2/M checkpoint is initiated through ATM activity. Joiner and colleagues have shown that inhibition of ChK1 and Chk2, two proteins integral to the G2/M transition, can influence the cell-cycle response to low-dose radiation. It is believed that failure of the cell to repair DNA damage in G2-phase cells leads to increased apoptosis. Nonetheless, inhibition of ChK1 and ChK2 also lead to IRR at radiation doses > 0.2 Gy. This is consistent with reports indicating that low dose radiation can stimulate repair of DNA damage. Interestingly, low dose radiation can also stimulate antioxidant capacity, apoptosis, and induction of immune responses, which collectively may provide effective local tumor control. In addition, hypoxia and nitric oxide levels can also affect cells sensitivity to radiation. Reduction of nitric oxide level enhances the radiosensitivity of hypoxic non-small cell lung cancer. Therefore, the identification of cellular pathways that are responsive to low dose radiation and their contribution to chemopotentiation is highly significant because this will provide a better measurement of the therapeutic response and contribute to the rational design of mechanism-based clinical trials.