p53-binding protein 1 (53BP1) is a critical regulator of cellular response to DNA double-strand breaks (DSBs)1
. To accomplish its repair function, 53BP1 must be recruited to the chromatin surrounding DSB sites that carry H4 methylation at Lys20 and H2A ubiquitination at Lys15. The structural basis of this recognition process was recently revealed by the complex structure of 53BP1 bound to a nucleosome core particle (NCP) containing Lys20-dimethylated H4 (H4K20me2) and Lys15-mono-ubiquitinated H2A (H2AK15monoUb). It is fascinating to note that ubiquitin ligase RNF168 ubiquitinated H2A not only on Lys15, but also on Lys13 without selectivity, and H2A bearing the K15Q mutation was still poly-ubiquitinated at Lys13 in vivo. This leads to two questions. First, is 53BP1 also a reader of H2A Lys13 ubiquitin mark? Second, is poly-ubiquitination redundant at the 53BP1 recruitment event?