DNA ligase IV (EC 6.5.1.1), which is conserved in all eukaryotes, is part of a family of ATP-dependent DNA ligases that are involved in DNA replication, recombination and repair. It is a nuclear enzyme that joins the breaks in the phosphodiester backbone of DNA by the process of non-homologous end joining (NHEJ). DNA ligases have two common domains: a catalytic domain (CD) that contains several conserved nucleotide-binding motifs, and a conserved non-catalytic domain (NCD). In addition, DNA ligase IV has a long C-terminal extension comprising of two BRCT domains (after the C-terminal domain of a breast cancer susceptibility protein, BRCA1), which are phosphopeptide-binding modules found in many proteins that regulate DNA damage responses (such as BRCA1, MDC1 and BARD1). These BRCT domains are connected to a short linker region that is required for the binding of the XRCC4 protein, which is important for ligase activity.