Nucleus {ECO:0000269|PubMed:10848582, ECO:0000269|PubMed:12446782, ECO:0000269|PubMed:15572677}. Note=In some cells it may be found in typically 1 or 2 discrete nuclear aggregates of unknown function which also contain TP53BP1. Also found in multiple discrete nuclear foci which increase in number following treatment with ionizing radiation or interstrand cross-linking agents. These foci overlap with those formed by the MRN complex (composed of MRE11, RAD50 and NBN) and BRCA1.