Measurement of fluorescence signals at various time points after SybrGreen addition. PBMC were exposed to an X-ray irradiation dose of 0.47 Gy (P1) or 2.3 Gy (P2). Cells in T and P0 samples were not irradiated. Note that the fluorescence signal decreased with increasing irradiation dose, as expected. Fluorescence intensity during the first 20 min upon SybrGreen addition remained constant but decreased thereafter. The ratios P0/T, P1/P0 and P2/P1 remained constant over the whole observation period.