Double‐strand break (DSB) is a critical DNA damage induced by ionizing radiation, and which is an actuating signal triggering cancer cell death in tumor radiotherapy. In mammalian cells, nonhomologous end‐joining (NHEJ) and homologous recombination (HR) are the two main pathways of DNA DSB repair, and both cooperate and compete with each other to promote efficient repair. Many regulatory factors play essential roles in the DSB repair pathway. For example, the MRE11–RAD50–NBS1 (MRN) complex and the Ku70/80 heterodimer/DNA–PKcs (DNA–PK) com...