P53-binding protein 1 (53BP1) is a multi-functional double-strand break repair 
protein that is essential for class switch recombination in B lymphocytes and 
for sensitizing BRCA1-deficient tumours to poly-ADP-ribose polymerase-1 (PARP) 
inhibitors. Central to all 53BP1 activities is its recruitment to double-strand 
breaks via the interaction of the tandem Tudor domain with dimethylated lysine 
20 of histone H4 (H4K20me2). Here we identify an uncharacterized protein, Tudor 
interacting repair regulator (TIRR), that directly binds the tandem Tudor domain 
and masks its H4K20me2 binding motif. Upon DNA damage, the protein kinase 
ataxia-telangiectasia mutated (ATM) phosphorylates 53BP1 and recruits 
RAP1-interacting factor 1 (RIF1) to dissociate the 53BP1-TIRR complex. However, 
overexpression of TIRR impedes 53BP1 function by blocking its localization to 
double-strand breaks. Depletion of TIRR destabilizes 53BP1 in the 
nuclear-soluble fraction and alters the double-strand break-induced protein 
complex centring 53BP1. These findings identify TIRR as a new factor that 
influences double-strand break repair using a unique mechanism of masking the 
histone methyl-lysine binding function of 53BP1.