ssDNA (BrdU foci) in NHEJ deficient cells. M059J (NHEJ deficient) and M059K (NHEJ proficient) cells were fixed 4 h after irradiation. (A) BrdU corresponding to ssDNA was detected with immunofluorescence, and (B) cells were scored as belonging to one of the following categories: I. Weak staining in the whole nucleus; II. Increased staining in the nucleus compared to I, dots are present in the whole nucleus; III. Strong staining in the nucleus, many dots with high intensity in the whole nucleus. At least 100 cells in three experiments were scored. Error bars represent the SD.