In order to investigate the relationships among CHEK2 mutations, family history of breast cancer, and the risk of breast cancer, Cybulksi and associates conducted a prospective study that included 7,494 BRCA1 mutation–negative patients with breast cancer and 4,346 control women. Family history of breast cancer in first-degree and second-degree relatives was considered. Study participants were genotyped for 4 founder mutations in CHEK2. A total of 227 patients with breast cancer (3%) and 37 controls (0.8%) had a truncating CHEK2 mutation (odds ratio [OR], 3.6; 95% CI, 2.6–5.1). Women who had a first- or second-degree relative with breast cancer had a higher OR (OR, 5.0; 95% CI, 3.3–7.6) than those with no family history of the disease (OR, 3.3; 95% CI, 2.3–4.7). The OR was 7.3 (95% CI, 3.2–16.8) if both a first- and second-degree relative had breast cancer. Assuming a 6% baseline risk of breast cancer (the general population risk in Poland), the lifetime risk of breast cancer for a woman with a CHEK2 mutation was estimated to be 20% if there was no family history of breast cancer, 28% if a second-degree relative had breast cancer, 34% if a first-degree relative had breast cancer, and 44% if both a first- and second-degree relative had breast cancer. The study was published in the August 29 online issue of the Journal of Clinical Oncology. Although gene mutations and baseline risks of breast cancer differ across countries, these results suggest that CHEK2 gene mutations do contribute to some cases of breast cancer, and that risk may be particularly high for women who have both a CHEK2 gene mutation and a family history of the disease.