The invention relates to a method of characterizing the cellular reaction of a cell sample to ionizing radiation, the cell sample having been obtained from cells taken from a non-irradiated or slightly irradiated region of a patient, and the method involving the following steps: (i) amplifying the sampled cells, the amplified cells constituting "the cell sample" (ii) determining on the cell sample the mean number of nuclear foci obtained with at least two of the markers pH2AX, pATM and MRE11 at observation times t (these mean numbers being designated N