There is interest in testing the hypothesis that the non-carrier sisters of women with a breast cancer 1 (BRCA1) mutation face a greater than expected risk of breast cancer. Genetic testing on 3568 women with breast cancer under the age of 50 years was performed. These cases were unselected for family history. Genetic testing was offered to 261 sisters of 188 mutation-positive cases. One of 72 mutation-negative sisters was diagnosed with breast cancer. Of the 17 sisters diagnosed with breast cancer, only one was a phenocopy. Thus, we are unable to confirm the hypothesis that the non-carrier sisters of mutation carriers face a risk of breast cancer beyond that of the general population.

In the process of counselling women from families with a known breast cancer 1 (BRCA1) mutation, we often encounter healthy women who are found to have a negative test. This is usually a source of great relief for the woman, and traditionally the genetic counsellor or physician offers the opinion that the residual risk is low—that is, approximately that of women in the general population. In Canada and the USA, the risk to the population is approximately 8% to age 75 years, and in Poland it is about 5%. In a recent paper from the UK, Smith and colleagues suggest that the residual risk is, in fact, much greater than this. They estimate that the risk of breast cancer …