Ionizing radiation (IR) induces single-strand breaks, i.e., cleavage of the phosphodiester backbone. When two single-strand breaks occur within approximately 10 base pairs, a DNA double-strand break (DSB) results. IR-induced DSBs are complex DNA damage lesions, frequently containing base damage, 5'-OH groups, and 3'-hydroxy or phosphoglycolate groups that must be removed prior to ligation in the final step of NHEJ (Friedberg et al, 1995; Nikjoo et al, 2001; Valerie and Povirk, 2003). The Ku70/80 heterodimer (XRCC5:XRCC6) (Walker et al., 2001) binds to the ends of the double-strand break. Ku can translocate inwards from the site of the break in an ATP-independent manner (reviewed in Dynan and Yoo, 1998). Binding of XRCC5:XRCC6 to DNA DSBs competes away the MRN complex and associated proteins from the DNA DSB (Sun et al. 2012).