MacPhail, S. H. and Olive, P. L. RPA Foci are Associated with Cell Death after Irradiation. Radiat. Res. 155, 672-679 (2001). Complexes containing replication protein A (RPA) were observed in human TK6 and WIL-2NS lymphoblast cells and SiHa cervical carcinoma cells exposed to 250 kV X rays. Image analysis of individual cells with fluorescence-tagged anti-RPA antibodies was used to measure numbers of discrete foci per cell. RPA foci formed in S-phase cells in response to radiation doses as low as 0.5 Gy, and the number of foci/nucleus was linearly related to dose up to 50 Gy. The maximum number of cells with foci occurred 4-8 h after exposure to 4 Gy, and subsequently declined. However, the number of RPA foci per nucleus (in those cells with foci) reached a maximum after 2-4 h. Apoptotic nuclei from irradiated TK6 and WIL-2NS cells initially contained foci, but these were lost as degradation continued. Radiation-induced micronuclei in SiHa cells were greatly enriched for RPA foci, and cells with nuclei without foci often contained micronuclei with multiple RPA foci. In SiHa cells examined up to 7 days after 4 Gy, RPA foci reappeared in one or more cells in up to 90% of the surviving colonies, and some cells contained 150 or more distinct foci. Reappearance of these complexes could be indicative of radiation-induced genomic instability. These results are consistent with the idea that RPA foci observed several hours after irradiation represent irreparable lesions and as such might be useful in identifying radiosensitive cells.