The NBS1/p95 protein has a pivotal role in the sensing and repair of chromosome breaks. A missense mutation in the NBS1 gene, I171V, has recently been associated with a ninefold increased risk of breast cancer in Polish patients. Positive associations have also been reported for leukaemia and larynx cancer suggesting that I171V could be a more general susceptibility factor for malignancies. We investigated the prevalence of this mutation in two large hospital-based case-control series from Germany and from the Republic of Belarus. The I171V substitution was detected in 20/1,636 Byelorussian breast cancer patients and in 18/1,014 Byelorussian controls (OR: 0.68; 95%CI: 0.36-1.30, P=0.3). The I171V substitution was furthermore detected in 10/1,048 German breast cancer patients and in 7/1,017 German controls (OR: 1.39; 95%CI: 0.53-3.67, P=0.7). There were no significant differences between I171V carriers and non-carriers among the cases with regard to age at diagnosis, family history or bilateral occurrence of disease. A meta-analysis of all hitherto available studies did not reveal a difference in the prevalence of I171V between breast cancer cases and controls (OR: 1.05; 95%CI: 0.64-1.74, P=0.9). We conclude that the I171V substitution is unlikely to constitute a strong risk factor for breast cancer in our study populations.