The gamma radiation (ionizing irradiation; IR) was delivered by a Theratronic cobalt unit T-1000 (Theratronic International, Inc., Ottawa, Canada) at a dose rate of 1.1Gy/min (SSD = 57.5cm). To evaluate the cell proliferation rate we seeded cells on 24-well plates at a density of 2610 4 cells/well. Cells were seeded 24 hours after IR and then they were analyzed by methyle thiazol tetrazolium assay (MTT assay, Sigma-Aldrich, St. Louis, MN). The amount of MTT formazon product was determined by using a microplate reader and the absorbance was measured at 560 nm (SpectraMax 250, Molecular Devices, Sunnyvale, CA).