Z-projection reveals irradiation pattern. Hela cell stably transfected with a plasmid coding for a GFP- tagged version of the protein 53BP1 irradiated with 20 MeV protons (LET = 2.6 keV at cell position). 16 protons were applied per point in a 5 × 5 μm matrix pattern. Approximately 30 min after irradiation a series of images with defined z-distance through the whole nucleus was recorded (z-Stack). The images were deconvoluted to remove the blur caused by out-of- focus signals and a z-projection was performed. The radiation-induced 53BP1 foci are marked with arrows.