3.3.1. The hypersensitivity to low-doses (HRS) effect The number of radiation-induced DNA damage, directly linked to physical energy deposition, is systematically proportional to the dose. Conversely, repair and signaling activities and clonogenic sur-vival are generally not linearly linked to the dose. The phenomenonof hypersensitivity to low dose (HRS) followed by induced radiore-sistance (IRR) is a representative example of a non-linearly dose-dependent event In fact, HRS is generally observed between 1 mGy and 50 cGy with a maximum peak at a cell-line.-dependent threshold of about 10–30 cGy. At this peak, clonogenic survival mayreach similar values as for doses of 1–3 Gy (i.e. at dose about 100times higher!). Although a number of molecular models have beenproposed, mechanisms of HRS are however unclear. It has beensuggested that HRS may depend upon changes in chromatin con-formation, failure of the ATM-dependent G2/M checkpoint, or DNArepair defects. More recently, we have shown that HRS may beexplained by the lack of recognition of DSB by the repair and sig-naling pathway. In fact, all the irradiation conditions where HRS isobserved correspond to exposure duration of less than 30 s duringwhich the DSB recognition does not seem to occur (Thomas et al.,2013). Interestingly, during our investigations, cells with an unex-pectedly high number of DNA breaks have been observed between1 mGy to 30 cGy. These so called “highly damaged cells” might correspond to the multi-aberrant cells observed incytogenetics. Investigations are therefore needed to explain howand in what conditions HDC phenomenon is linked to HRS andwhat are the molecular and cellular mechanisms behind the occur-rence of these specific low-dose events. Two types of data suggest that chromatin organization influences HRS effect: (1) inhibitors of the polyPolymerase that is required for chromatin integrity, would facilitate the occurrence of HRS