3.3.1. The hypersensitivity to low-doses (HRS) effectThe number of radiation-induced DNA damage, directly linkedto physical energy deposition, is systematically proportional to thedose. 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 (Fig. 5). In fact, HRS is generally observed between1 mGy and 50 cGy with a maximum peak at a cell-line-dependentthreshold 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” (HDC)might correspond to the multi-aberrant (rogue) 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 suggestthat chromatin organization influences HRS effect: (1) inhibitors ofthe poly(ADP-ribose) Polymerase (PARP) that is required for chro-matin integrity, would facilitate the occurrence of HRS (Chalmers