Seventy-two hours after irradiation, cells and media were collected, centrifuged, and resuspended in Annexin binding buffer with cell density adjusted to ~106/ml. Cells were stained with propidium iodide (Sigma-Aldrich) and Annexin V-Cy5 following the manufacturer’s protocol (BioVision, Milpitas, CA), and then analyzed by a LSRII flow cytometer (BD Biosciences, San Jose, CA). Senescence-associated β-galactosidase staining was performed using a commercial kit (Cell Signaling, #9860) as described (23 (link)).