The major genetic risk factor for ovarian cancer is a mutation in "BRCA1" or "BRCA2" genes, or in DNA mismatch repair genes, which is present in 10% of ovarian cancer cases. Only one allele need be mutated to place a person at high risk. The gene can be inherited through either the maternal or paternal line, but has variable penetrance. Though mutations in these genes are usually associated with increased risk of breast cancer, they also carry a substantial lifetime risk of ovarian cancer, a risk that peaks in a person's 40s and 50s. The lowest risk cited is 30% and the highest 60%. Mutations in BRCA1 have a lifetime risk of developing ovarian cancer of 15–45%. Mutations in "BRCA2" are less risky than those with "BRCA1", with a lifetime risk of 10% (lowest risk cited) to 40% (highest risk cited). On average, BRCA-associated cancers develop 15 years before their sporadic counterparts because people who inherit the mutations on one copy of their gene only need one mutation to start the process of carcinogenesis, whereas people with two normal genes would need to acquire two mutations.