Recruitment of XRCC1 and PCNA at DNA damage sites in living cells. (A) Schematic representation of the fluorescent fusion proteins. (B) Live cell imaging of a microirradiated Hela cell coexpressing GFP-XRCC1 and RFP-PCNA. Accumulation of GFP-XRCC1 can be observed immediately after microirradiation, while RFP-PCNA accumulates with a short delay of about 2–10 s (indicated by arrows). (C) Quantitative evaluation of recruitment kinetics showing mean curves. Error bars represent the standard error of the mean. Immediate and fast recruitment of GFP-XRCC1 precedes slow and constant recruitment of RFP-PCNA at DNA damage sites. Scale bar, 5 μm.