The BRCT regions are two repeating structures in BRCA1 at the carboxyl-terminus and are ubiquitous in some proteins involved in cell cycle checkpoint and in DNA repair. Here, using electron microscopy, we show direct evidence that the BRCT regions of BRCA1 bound double-strand breaks of DNA. The BRCT regions could multimerize thus forming large protein particles. Smeared patterns of DNA fragments were consistently shown in the gel retardation assay. A single BRCT was sufficient for DNA binding. The smeared patterns were also observed in BRCTs of TopBP1, suggesting: that multimerization may be an important feature of BRCTs. The recombinant second BRCT of XRCC1 (X-ray repair cross-complementing group 1), whose folding was determined by X-ray crystallography, also showed similar DNA end binding images. It is possible that some BRCTs are fundamental structures that detect DNA damages.