Nonhomologous end joining—the importance of end tethering and beyond

The nonhomologous end joining (NHEJ) pathway is a major mechanism for repairing DNA double-strand breaks (DSBs), essential for lymphocyte development and for maintaining genome stability in postmitotic cells, including neurons. Under pathological conditions, NHEJ is also responsible for most oncogenic translocations and the repair of DSBs induced by chemotherapy/radiation. Advances in structural biology and single-molecule imaging have captured NHEJ at various stages of the reaction, revealing a modular, dynamic organization with built-in redundancy and resilience. Here, we describe NHEJ in five phases (end sensing, end protection, end tethering, end processing, and end ligation), highlighting their molecular features and transitions and how structural insights explain genetic interactions and physiological consequences, including lymphocyte development. Unlike other DSB repair mechanisms, NHEJ operates with minimal or no sequence complementarity, relying instead on protein assemblies to bridge and stabilize the two DNA ends. This review highlights the unique end-tethering phase, supported by at least five known overlapping machineries, all anchored on Ku.