In a very large European study of female BRCA1 and BRCA2 mutation carriers that attempted to correct some of the methodological limitations of prior studies, exposure to diagnostic radiation including mammography before the age of 30 was associated with an increased risk of breast cancer (HR 1.9, 95% CI 1.2 to 3.0), at dose levels significantly lower than those at which increased risk has been found in non-carriers. The gene discussed is BRCA1; the disease is breast cancer.