Figure 4. Levels of 53BP1 and formation of IRIF in cycling cells. Immunofluorescence with 53BP1 and Cyclin A antibodies was performed in asynchronously growing cells that were irradiated with 8 Gy or mock irradiated. DAPI staining was used to identify all nuclei. Cyclin A positive cells represent cells in S/G₂ phases of the cell cycle, while Cyclin A-negative cells represent cells in G₁. Graphs show quantitation of the intensity of fluorescence (relative fluorescence units) in 200 cells per condition. We compared intensity of fluorescence of 53BP1 between Cyclin A-positive (S/G₂) and Cyclin A-negative (G₁) cells that were irradiated with 8 Gy. Statistically significant differences were not observed (NS).