Mobility of XRCC1 and PCNA at DNA damage sites. (A) Two separate subnuclear spots of a transiently transfected Hela cell were microirradiated. The mobility of accumulated fluorescent fusion proteins was determined by bleaching one of the two spots 5 min after microirradiation and subsequent recovery measurements. Inset shows the bleached microirradiated site. Scale bar, 5 μm. (B) FRAP data from 11 individual experiments are shown as mean curves. Error bars represent the standard error of the mean.