Figure 3. Human cells contain more 53BP1 foci following DNA damage. (A) Mouse fibroblasts and human fibroblasts were seeded onto coverslips and treated with 1.5 ug/mL neocarzinostatin for 2 hours. Cells were fixed either right after 2 hours of damage or at 6, 24, 48, or 72 hours following damage and stained for 53BP1 foci. At least 400 cells per time point were counted, and untreated mouse and human fibroblasts were included as controls. Images were analyzed using Slidebook software equipped with deconvolution capabilities. (B) Western blot analysis of 53BP1 levels in mouse and human cells. Mouse and human cells were subjected to DNA damage as in A and allowed to recover for 72 hours when 53BP1 levels began to decline by immunofluorescence. Total protein was extracted and examined for the relative level of 53BP1 compared to untreated controls. β-actin was included as a loading control. (C) Quantification of 53BP1 foci formation in mouse and human cells is presented. Cells scored for 53BP1 foci in A were grouped according to the number of foci per cell: 5-10 foci, 10-20 foci, or >20 foci per cell.