Ku is a protein that binds to DNA double-strand break ends and is required for the non-homologous end joining (NHEJ) pathway of DNA repair. Ku is evolutionarily conserved from bacteria to human. The ancestral bacterial Ku is a homodimer (two copies of the same protein bound to each other).

Eukaryotic Ku is a heterodimer of two polypeptides, Ku70 (XRCC6) and Ku80 (XRCC5), so named because the molecular weight of the human Ku proteins is around 70 kDa and 80 kDa. The two Ku subunits form a basket-shaped structure that threads onto the DNA end. Once bound, Ku can slide down the DNA strand, allowing more Ku molecules to thread onto the end.