 Residues arginine 64, leucine 65, and leucine 115 in Cer-XLF interact with lysines 65 and 99 in XRCC4 within their N-terminal domains.Together they form a filament bundle that wraps around DNA in an alternating pattern.Hyper-phosphorylation of the C-terminal alpha helical domains of XRCC4 by DNA-PKcs facilitates this interaction.