Reports

53BP1 is a multi-domain DNA damage response factor containing a chromatin-binding tudor domain, an oligomerization domain, tandem BRCA1 C-terminal (BRCT) domains, and an N-terminal domain with 28 SQ/TQ potential phosphorylation sites for phosphatidylinositol 3-kinase-related kinases [PIKKs, ATM/ATM and Rad3-related (ATR)/DNA-dependent protein kinase catalytic subunit (DNA-PKcs)] (1–3). 53BP1 contributes to DNA repair in several ways: it facilitates joining between intrachromosomal DSBs at a distance (synapsis) (4–7); it enables hetero-chromatic DNA repair through relaxation of nucleosome compaction (2, 3), and it protects DNA ends from resection and thereby favors repair of DSBs that occur in G1 by non-homologous end joining (NHEJ) (4, 5, 8). Consistent with its role in DNA end protection, 53BP1 is essential for CSR in B lymphocytes (9, 10). Structure-function studies indicate that, besides its recruitment to DNA ends, protection requires 53BP1 phosphorylation (4), but how thi