The specific binding site for hRad50 on the BRCA1 protein was found to be a fragment containing amino acids 341 to 748. The amino-terminal half of hRad50 was required for BRCA1 binding. Treatment of T24 cells with gamma irradiation or MMS did not appear to alter the amount of BRCA1~hRad50 complex, suggesting that it exists even in the presence of DNA damage. Both BRCA1 and hRad50 display discrete nuclear foci after treatment of cells with genotoxic agents. Among cells displaying both hRad50 and BRCA1 foci after gamma irradiation, >90% showed substantial co-localization. Co-localization of BRCA1 and hRad51 has also been reported, and these two types of foci appear to be mutually exclusive. Radiation-induced hMre11 and p95 foci were also shown to co-localize with BRCA1 and hRad50 (with which they form a complex). BRCA1, hRad50, hMre11 and p95 irradiation-induced foci were all found to be diminished in HCC1937 breast cancer cells, which express a carboxy-terminally truncated BRCA1 protein. Transfection of wild-type BRCA1 into the HCC1937 cells restored these foci, implicating BRCA1 in the defective response of these proteins in HCC1937 cells. The truncated BRCA1 protein, present in detectable levels in these cells, was found to be absent from the hRad5~hMre11~p95 complex. HCC1937 cells were found to be hypersensitive to MMS compared with T24 and MCF-7 cells, both of which express the full-length BRCA1 protein. Transfection of wild-type, but not mutant, BRCA1 substantially increased survival of MMS-treated HCC1937 cells. In contrast, transfection with wild-type BRCA2 did not affect cell survival under the same conditions.