Repair orders

A large and varied group of proteins accumulates at the site of double strand DNA breaks (DSBs). But just what role each plays in the assembly process and in what order they arrive has been unclear. Bekker-Jensen et al. clear up one controversy on page 201, demonstrating that Mdc1/NFBD1 precedes 53BP1's arrival and is required for 53BP1's stable association with the chromatin surrounding DSBs.

Mdc1 and 53BP1 are two of the earliest proteins to accumulate at DSBs, but recent studies reported conflicting data as to how the proteins influence each other's binding at damaged chromosomes. To find out, Bekker-Jensen et al. followed the assembly process in real time. They used a micro-laser to generate DSBs in tissue culture cells and started time-lapse imaging immediately.

GFP-tagged 53BP1 and Mdc1 began to accumulate at the sites...