Tumor protein p53 binding protein (53BP1) has been identified in a yeast two-hybrid screen as a protein that interacts with the central DNA–binding domain of p532. Similar to breast cancer susceptibility gene 13, 4 (BRCA1; 53BP1 enhances p53-dependent transcription5. Interestingly, the COOH terminus of 53BP1 contains tandem BRCA1 COOH terminus (BRCT) motifs.

53BP1 becomes hyperphosphorylated and rapidly relocates to the sites of DNA strand breaks in response to ionizing radiation. 53BP1 foci formation is reduced in ATM-deficient cells and can be inhibited by wortmannin in ATM wild-type cells. Moreover, radiation-induced hyperphosphorylation of 53BP1 is absent in cells treated with wortmannin, as well as in ATM-deficient cells. 53BP1 participates in DNA damage–signaling pathways and is regulated by ATM after γ-radiation6.