 Cer-XLF only binds to XRCC4 without direct interaction with LigIV.LigIV then joins the DNA ends by catalyzing a covalent phosphodiester bond.V(D)J recombination is the rearrangement of multiple, distinct gene segments in germ-line DNA to produce the unique protein domains of immune cells, B cells and T cells, that will specifically recognize foreign antigens such as viruses, bacteria, and pathogenic eukaryotes.