Abstract: Objective 　The experiment investigated the gamma irradiation sensitivity of B-lymphocyte cell line from breast cancer patient carrying a BRCA1 mutation. The aim was to get some information to establish an in vit ro experiment system for studying the mechanisms associated with tumor initiation and progression. Methods 　The cells were exposed 60 Co and followed 28hs post-irradiation culture with RPMI1640 with Cytochalasin B. CBMN was employed to analyze irradiation sensitivity of treated cells. Results 　The frequencies of MNed BNC and NPB were significantly increased in both cell lines. No significant differences on above biomarkers with respect to spontaneous and radiation induced damage between tested and cont rol cell line were found. Conclusion 　BRCA1 mutation is not very important to chromosomal radiosensitivity. There was no significant decrease of genomic stability in cells with BRCA1 mutation in our in vitro system.