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.