Single-stranded DNA-dependent ATP-dependent helicase. Involved in DNA non-homologous end joining (NHEJ) required for double-strand break repair. When associated with KU70, binds to double-stranded telomeric and non-telomeric DNA sequences, but not to single-stranded DNA. Plays a role in maintaining telomere length. Acts as a negative regulator of telomerase. Binds to and recombines double-stranded T-DNA molecules. ATP + H2O = ADP + H(+) + phosphate Heterodimer with KU70. Interacts with WEX. Predominantly in the nucleus. Expressed ubiquitously. Up-regulated in response to induction of double-strand breaks. Down-regulated by heat shock. No visible phenotype when grown under normal conditions. Hypersensitivity to ionising radiation (IR) and to DNA-damaging agents. Longer telomeres. Defective in T-DNA integration. Belongs to the ku80 family.