MDC1 is SUMOylated at lysine-1840 with SUMO2,3 (Luo et al. 2012, Hendriks et al. 2014, Tammsalu et al. 2014). SUMOylation is required for degradation of MDC1. SUMOylation of MDC1 is required for recruitment of RNF4 (Yin et al. 2012), which is believed to ubiquitinylate MDC1, resulting in degradation of MDC1.