U2OS cells were split into two aliquots and cultured in one of the following media (DMEM + 10 % dialysed FCS): i) light medium: contains Arg-0 and Lys-0 ii) heavy medium: contains Arg-10 and Lys-8 Cells were passaged at least three times in the respective medium to ensure complete labelling of proteins. SILAC experiments were performed in a forward and a reverse reaction: for the forward experiment, heavy-labelled cells were irradiated with 20 Gy and harvested 45 minutes after irradiation, while light-labelled cells were left untreated. In the reverse experiment, labels were swapped and light-labelled cells were irradiated, while heavy labelled cells were left untreated.