 In order for XRCC4 to be sequestered from the cytoplasm to the nucleus to repair a DSB during NHEJ or to complete V(D)J recombination, post-translational modification at lysine 210 with a small ubiquitin-related modifier (SUMO), or sumoylation, is required.SUMO modification of diverse types of DNA repair proteins can be found in topoisomerases, base excision glycosylase TDG, Ku70/80, and BLM helicase.A common conserved motif is typically found to be a target of SUMO modification, ΨKXE (where Ψ is a bulky, hydrophobic amino acid).