Subject to radiation-induced DNA damages such as double-strand breaks (DSB), cells reacts by triggering a whole series of phosphorylation events coordinated within multi protein complexes whose interplay is still misknown (DNA damage response i.e. DDR). Indirect Immunofluorescence cell scanning has revolutionized radiation biology research by permitting the detection of individual DSB in each cell nucleus in a dose range from 1 mGy to 10 Gy. This technique has notably confirmed that yield of unrepaired DSB is correlated with cell RS. From a broad spectrum of human radiosensitive skin fibroblast cell lines, the Inserm CRU 1052 team proposed a general model of DSB signaling and repair and a molecular assay to stratify patients according to their individual RS.