X-ray repair cross-complementing protein 5 (XRCC5), alternative name (Ku80), is single stranded DNA-dependent ATP-dependent helicase. It involves in chromosome translocation. Ku80 along with Ku70 forms a heterodimeric protein originally identified as autoantigens associated with several autoimmune diseases including scleroderma, polymyositis and systemic lupus erythematosus. Ku is widely expressed nuclear protein that binds to and stabilizes the ends of DNA at telomeres or double-stranded DNA breaks. The inhibition of Ku80 can damage the DNA repair pathway of tumor cells and potentially increase the killing of cancer cells, and reduce drug resistance.