(A) Cells were transfected with the indicated siRNAs, and localized DNA damage was induced using a focused laser microbeam. The cells were fixed 15 min or 2 hr later and stained with antibodies against γH2AX and K48-Ub. (B) The accumulation of K48-Ub was quantified according to the fluorescence intensity obtained using the corresponding antibody on top of γH2AX stripes. The immunoblot shows the extent of UBE4A depletion in tis experiment. (C) K48-Ub lines were classified as ‘strong’, ‘average’ or ‘weak/undetectable’. (D-F) Similar analysis as in (A-C) for K63-linked ubiquitin chains. AU - arbitrary units. Quantified data in B and C are represented as mean ± SEM (3 independent experiments, n>70), and in E and F as mean ± SEM (4 independent experiments, n>80). *p<0.05, ** p<0.005, *** p-<0.0005 (student's t-test, relative to the siCTRL). (See also Figures S6 and S7).