a number of “monogenic” studies that represent, still to date, the great major-ity of papers in radiobiology. A global and multigenic approach istherefore needed to better reflect the large spectrum of radiosen-sitivity reactions observed in clinic (Massart et al., 2009). Whenthe list of the most radiosensitive syndromes is established as afunction of survival fraction at 2 Gy (SF2) (Table 1), some interest-ing conclusions can be made: the great majority of radiosensitivesyndromes are associated with moderate radiosensitivity andnot hyper-radiosensitivity (that leads to death after radiotherapysession). All these genetic syndromes that a radiation oncologistcan encounter in his career are caused by mutations of genesinvolved in DSB repair and signalling but not necessarily