SUMOylation of MDC1 by PIAS4 creates a docking site for RNF4, an E3 ubiquitin ligase. RNF4 binds to MDC1 specifically when SUMO2 is attached to lysine K1840 of MDC1. RNF4 polyubiquitinates MDC1 through ubiquitin lysine residue K48 cross-linking. RNF4-mediated ubiquitination targets MDC1 for degradation and causes dissociation of MDC1-bound proteins from DNA double strand breaks (DSBs). While additional regulation steps may be involved, the activity of RNF4 is necessary for the initiation of resection of DNA ends at DSBs and progression of homologous recombination (Luo et al. 2012, Yin et al. 2012, Galanty et al. 2012).