RNF8/RNF168 and POH1 play opposing roles in 53BP1 accumulation. (A) Depletion of POH1 restores 53BP1 foci in cells depleted of RNF8 or RNF168. U20S transfected with Non-T, RNF8 or RNF168 siRNAs or co-transfected with RNF8/RNF168 siRNAs with POH1 siRNA and exposed to 2 Gy irradiation and fixed 1 h later before incubation with anti-53BP1 antibody. The white line shows the outline of the DNA stained by Hoechst. (B) Protein levels in POH1 and RNF8/168 siRNA-treated cells. U20S transfected with Non-T, POH1, RNF8, RNF168 siRNA or a combination with POH1 siRNA, lysed, immunoblotted with anti-53BP1, anti-RNF8 (left panel) or anti-RNF168 (right panel), anti-POH1 and anti-β-actin antibodies. (C) Quantification of cells with 53BP1 foci. U20S transfected with Non-T, RNF8 or RNF168 siRNA or siRNA to RNF8 and RNF168 and POH1 together, scored for the presence or absence of 53BP1 foci (>5 foci/cell) (100 cells/condition, 2 repeats).