The BRCA1 gene mutation is a mutation of a gene that suppresses cancer. Therefore, those mutations, when we can detect them, tell us that a persona is likely to develop cancer and can undertake some preventive measures, such as a preventive mastectomy.
It can be seen as an example of a disease that is predictable and which is changed through social practices - our social practices (prevention) change how this gene is expressed (it no longer necessarily lead to cancer)