DNA double-strand breaks (DSBs) are potentially hazardous lesions that can be induced by ionizing radiation (IR), radiomimetic chemicals, or DNA replication inhibitors. Melanoma associated antigen (mutated) 1 (MUM1, EXPAND1) is a PWWP-domain containing chromatin binding protein involved in maintaining chromatin architecture of interphase chromosomes. In response to DNA damage, EXPAND1/MUM1 accumulates at sites of DNA double strand breaks through direct interaction with DNA repair factor 53BP1. Accumulation of EXPAND1/MUM1 at damaged DNA sites is thought to modify the structure of the chromatin and allow access to other DNA repair factors. 53BP1 activates the checkpoint kinase ATM and promotes DNA double strand break repair via nonhomologous end joining (NHEJ) repair pathway.