When the association between XRCC1-399 genotypes and breast cancer risk was studied according to smoking habits, increased breast cancer risk with dose-response was seen among women who had ever smoked actively and carried either one (OR 2.14, 95% CI 1.15–3.97) or two (OR 3.27, 95% CI 1.25–8.58, p for trend = 0.003) XRCC1-399 Gln variant alleles compared to those carrying the Arg/Arg genotype (p for interaction between smoking habits and XRCC1-399 genotype 0.025) (Table 3). Here, XRCC1 is linked to breast cancer.