Cellular capacity to repair DNA damage is limited, partly due to a fact that the number of copies of nuclear proteins available for a task of DNA repair is limited. Two key players involved in repair of SSBs, XRCC1 (X-ray Repair Cross Complementing Protein 1), which is involved in short patch and long path repair of single-strand DNA breaks (SSBs), and PCNA (Proliferating Cell Nuclear Antigen), which is primarily involved in DNA replication, are recruited to the site of damage. Aims. This research is focused on understanding the process of saturation of a capacity to repair single-strand DNA breaks, by means of imaging recruitment of XRCC1 and PCNA to DNA lesions induced in non-replicating and replicating cells. The goal of this work was to assess cellular capacity to recruit detectable numbers of PCNA and XRCC1 molecules to several DNA lesions induced in close succession or separated by time interval, and quantitate a process of saturation of repair capacity as a function of the number of damage sites and the position of a cell in the cell cycle. Methods. Local DNA damage (SSBs) was induced by exposing a small region of the cell nucleus to a focused beam of laser light1. Live cells, expressing GFP-PCNA and RFP-XRCC1, or cells stained by immunofluorescence, were imaged using confocal fluorescence microscopy. Results. XRCC1 was recruited to the induced DNA lesions in all phases of the cell cycle, in contrast to PCNA which was not recruited in early S phase. Recruitment of PCNA was detected exclusively in middle and late S-phase. When DNA SSBs were induced at short time intervals (seconds) in several locations (from 1 to 30) in the cell nucleus, RFP-XRCC1 was recruited to only 5-6 of them. The recruitment of GFP-PCNA was limited to an even lower number of damage spots. When DNA lesions were induced in close succession, the amount of XRCC1 was the highest in the first spot, and lower in each subsequent location, suggesting that the cell activated recruitment very quickly, and was significantly exhausting the stock of the repair protein by recruiting it to each subsequent DNA lesion. Interestingly, a small amount of XRCC1 and PCNA which was not recruited to any damage site always remained in the cell nucleus outside of the damaged regions. Immunofluorescence studies confirmed the results obtained with live cells expressing fusion proteins. Conclusions. Cellular ability to repair single-strand DNA breaks that were generated in different locations, in succession of each other, is limited to a just few SSBs spots. The cell appears to recruit very large number of copies of XRCC1 and PCNA to the first detected lesion rather than spread the resources over all the lesions. In early and late S-phase, when the number of damage sites exceeds approximately 3, the available PCNA pool appears to be exhausted, while recruitment of XRCC1 is still detectable. This suggests switching repair mechanisms from long to short patch. Project funded by the National Science Centre, Poland, grant no. 2017/27/N/NZ1/00975 1. Solarczyk, K.J. et al. Inducing local DNA damage by visible light to study chromatin repair. DNA Repair (Amst). 11, 996–1002 (2012).