PIAS4 SUMOylates RNF168 at an unknown lysine residue (Danielsen et al. 2012). Both RNF168 and HERC2 are SUMOylated at double-strand breaks in DNA. SUMOylation of RNF168 is required for its retention at double-strand breaks.
SUMO transferase activity of PIAS4 [nucleoplasm]