Question: why brca1 causes breast cancer?
Answer: A small percentage of people (about one in 400, or 0.25% of the population) carry mutated BRCA1 or BRCA2 genes. A BRCA mutation occurs when the DNA that makes up the gene becomes damaged in some way. When a BRCA gene is mutated, it may no longer be effective at repairing broken DNA and helping to prevent breast cancer.