Our analyses of family data in 27 families of carriers of either protein-truncating (n = 11) or rare, evolutionarily unlikely, potentially damaging missense mutations (n = 16) demonstrated a significantly increased risk of breast cancer with a penetrance that appears similar to that conferred by germline mutations in BRCA2. However, even in a study of this size, the confidence intervals are wide. Here, BRCA2 is linked to breast cancer.