DNA double-strand breaks represent the most potentially serious damage to a genome, and hence, many repair proteins are recruited to DNA damage sites by as yet poorly characterized sensor mechanisms. We clarified that NBS1 physically interacts with gamma-H2AX to form nuclear foci at DNA damage sites. The fork-head associated (FHA) and the BRCA1 C-terminal domains (BRCT) of NBS1 are essential for this physical interaction and focus formation of NBS1 in response to DNA damage. The inhibition of this interaction by introduction of anti-gamma-H2AX antibody into cells abolishes NBS1 foci formation in response to DNA damage. Consequently, the FHA/BRCT domain is likely to have a crucial role for both binding to histone and for re-localization of the NBS1/hMRE11/hRAD50 complex to the vicinity of DNA damage. Moreover, the foci formation of DNA repair-related proteins containing BRCT domain, such as BRCA1, requires the interaction with gamma-H2AX in response to DNA damage. These findings indicate that the physical interaction between gamma-H2AX and DNA repair-related proteins is indispensable for the recruitment of these proteins. Further, it was recently reported that the NBS1/hMRE11/hRAD50 complex has a crucial role for both the recruitment of ATM to DNA damage sites and the subsequent activation of ATM. Therefore, both gamma-H2AX and the NBS1/hMRE11/hRAD50 complex might function for the initial recognition of DNA damage.