Whats cause and effect of brca1 gene on breast cancer?

Genetic mutation: Genetic mutation in long arm of chromosome 17 in brca 1 , in 13 brca 2, instead of normal protective proteins which control abnormal growth of cell , repair broken down dna, maintain genetic stability, the abnormal proteins fail to protect, and responsible for 40% inherited breast cancers, 80% of breast & ovarian cancers.

Defect in DNA repair: Brca1 is a large protein involved in repairing dna damage. Deficiency in this protein due to mutation of brca1 gene leads to defective repair of damage to dna that leads to uncontrolled proliferation of cells and cancer.