Sumoylation of MDC1 on K1840 is important for the HR DNA repair and DDR. (A, B) Effects of MDC1 sumoylation on HR (A) and NHEJ (B); Error bars represent s.e.m. from three independent experiments. **P<0.01 two-tailed Student’s t-test. (C) MDC1+/+, MDC1−/−, and MDC1−/− MEFs reconstituted with WT or K1840R MDC1 were irradiated and colony formation assay was performed. Error bars represent s.e.m. from three independent experiments. (D, E) MDC1−/− MEFs reconstituted with WT or K1840R FLAG-MDC1 were infected with Ctrl or RNF4 shRNA. The cells were subjected to HR (D) or NHEJ (E) assay as described in method. Error bars represent s.e.m. from three independent experiments. **P<0.01, *P<0.05 two-tailed Student’s t-test. (F) Control cells and cells depleted RNF4 with shRNA were irradiated and colony formation assay was performed. Error bars represent s.e.m. from three independent experiments. (G) MDC1−/− MEFs reconstituted with WT or K1840R MDC1 were stably depleted RNF4 with shRNA or control shRNA. Cells were irradiated and colony formation assay was performed. Error bars represent s.e.m. from three independent experiments.