Complex sentence: The major genetic risk factor for ovarian cancer is a mutation in "BRCA1" or "BRCA2" DNA mismatch repair genes, which is present in 10% of ovarian cancer cases.
Split into simpler sentences: The major genetic risk factor for ovarian cancer is a mutation in "BRCA1" or "BRCA2" DNA mismatch repair genes. This mutation is present in 10% of ovarian cancer cases.