Immunochemical detection of endogenous XRCC1 and PCNA at DNA repair sites. Widefield fluorescence images of Hela cells are shown. Cells were fixed at indicated time points after laser microirradiation. (A) Both, XRCC1 and PCNA, accumulate at laser-induced DNA damage sites. (B) Microirradiated Hela cells were extracted with 0,5% Triton-X100 and 500 mM NaCl prior to fixation. After in situ extraction no endogenous XRCC1 can be detected at microirradiated sites while PCNA accumulations can still be observed. Scale bars, 5 μm.