Rapid replication fork breakage (RRFB) is independent of MUS81.

<p>(A) Western blot of whole cell extracts of RKO cells showing depletion of Mus81 by siMus81 compared to a control, siLUC. Loading: nonspecific band. (B) Representative fields of QIBC images after MUS81 depletion and treatment with 4 mM HU for 4 hours. (C) Quantification of nuclear γH2AX (left) or p53BP1 (right) fluorescence intensity by QIBC in >1500 S-phase cells depleted of MUS81 (siMUS81) or depleted with a control siRNA (siLUC) and either treated or not treated with 4mM for 4 hours. ns: not significant (Mann–Whitney test). (D) Quantification of DSBs in cells depleted of MUS81 (siMUS81; blue) or depleted with a control siRNA (siLUC; gray) by neutral comet assay. Bars correspond to the median tail moment (% DNA in tail x tail length). Error bars represent standard deviations from 3 independent experiments in which at least 150 comets were scored. ns: not significant, ***p < 0.001 (Student’s T test).