Repair protein and DNA. Molecular model of the Ku heterodimer (purple and brown) bound to a strand of DNA (deoxyribonucleic acid, yellow and green) as part of the repair process. The Ku protein (a heterodimer) consists of two subunits, Ku70 and Ku80. It forms a ring to surround the DNA strand, and a cradle to stabilise it. The shape fits the grooves of DNA, helping to bring together the ends of a broken strand, allowing repair to take place at the site of the break.