There is no single “breast cancer gene.” Several genes are linked to breast cancer risk, but the two most significant are BRCA1 and BRCA2. Women who inherit a harmful mutation in either gene have a greater than 60% chance of developing breast cancer in their lifetime. That said, only 5% to 10% of all breast cancers are caused by inherited gene mutations. The majority arise from a combination of aging, hormonal factors, and random genetic errors that accumulate over a person’s life.
BRCA1 and BRCA2: The Highest-Risk Genes
BRCA1 and BRCA2 are the genes most strongly associated with hereditary breast cancer. Both are tumor suppressor genes, meaning their normal job is to help your cells fix damaged DNA. When either gene carries a harmful mutation, that repair system breaks down, and damaged cells are far more likely to become cancerous.
The risk numbers are substantial. By age 80, women with a BRCA1 or BRCA2 mutation face roughly a 74% cumulative risk of developing breast cancer. These mutations also raise the risk of ovarian cancer: 39% to 58% lifetime risk for BRCA1 carriers, and 13% to 29% for BRCA2 carriers.
BRCA mutations are inherited in an autosomal dominant pattern, which means you only need one copy of the faulty gene (from either parent) to have an elevated risk. Each child of a carrier has a 50% chance of inheriting the mutation. These mutations are found across all ethnic groups but are notably more common in people of Ashkenazi Jewish descent.
How BRCA Mutations Actually Cause Cancer
Your DNA gets damaged constantly, from normal cell division, sun exposure, and everyday chemical processes. Healthy BRCA1 and BRCA2 proteins coordinate a precise repair process called homologous recombination, which fixes breaks in both strands of the DNA double helix. BRCA2 binds directly to a helper protein and guides it to the damaged site, while BRCA1 controls the signaling that keeps the repair process on track.
When these genes are mutated, the cell loses its most accurate repair tool. It falls back on a sloppier method that essentially glues broken DNA ends together without checking for accuracy. This introduces new mutations, particularly deletions of genetic material. Over time, errors pile up across the genome, including in genes that control cell growth. The result is genomic instability, a hallmark of cancer development. Cells with BRCA2 deficiency also develop problems with chromosome separation during division, producing structural abnormalities that accelerate the path toward a tumor.
Other High-Risk Genes: TP53 and PTEN
Two rarer gene mutations carry breast cancer risks that are even higher than BRCA, though they affect far fewer people.
Mutations in the TP53 gene cause Li-Fraumeni syndrome, a condition that dramatically raises the risk of multiple cancers beginning in childhood and young adulthood. For women with a TP53 mutation, the cumulative incidence of breast cancer reaches approximately 85% by age 60. About 31% of those women will also develop cancer in the opposite breast. Li-Fraumeni syndrome is rare, but it is one of the most aggressive hereditary cancer predispositions known.
Mutations in the PTEN gene cause Cowden syndrome, characterized by multiple noncancerous growths called hamartomas along with elevated cancer risk. Lifetime breast cancer risk for women with a PTEN mutation is estimated at 85%, with risk climbing steeply after age 50 (roughly 67% by age 60 and 77% by age 70).
Moderate-Risk Genes: PALB2, CHEK2, and ATM
Beyond the high-risk genes, several “moderate penetrance” genes meaningfully increase breast cancer risk without reaching the levels of BRCA or TP53. The three most well-studied are PALB2, CHEK2, and ATM, all of which play roles in DNA damage repair.
PALB2 sits at the top of this group. The predicted average breast cancer risk by age 80 for a PALB2 mutation carrier is about 50%, placing it closer to BRCA territory than the other moderate-risk genes. CHEK2 carriers face roughly a 30% risk by age 80, and ATM carriers about 28%. For comparison, the average woman’s lifetime breast cancer risk is around 13%. So even a “moderate” genetic risk roughly doubles or triples the baseline.
These genes are tested on most multigene panels today, and a positive result typically changes screening recommendations. You might be offered breast MRI in addition to mammography, or screening might start at a younger age.
Most Breast Cancer Is Not Inherited
Despite the attention given to genetic mutations, roughly 90% to 95% of breast cancers are not caused by a single inherited gene. These “sporadic” cancers develop from mutations that accumulate in breast tissue over a lifetime. Age is the single biggest risk factor: the older you are, the more time your cells have had to acquire random DNA errors.
About 10% to 30% of breast cancer cases have some hereditary component, meaning family history plays a role. But only 5% to 10% involve a clearly identifiable high-penetrance gene mutation like BRCA1 or BRCA2. The rest of the hereditary risk likely comes from combinations of many common genetic variants, each contributing a small amount of risk that is difficult to measure individually.
This means that most women diagnosed with breast cancer do not carry a BRCA mutation, and most women with a family history of breast cancer will not test positive for a known gene mutation. Family history still matters for screening decisions, but a negative genetic test does not mean zero risk.
How Genetic Mutations Shape Treatment
Knowing your genetic status does more than quantify risk. It can directly influence treatment if you are diagnosed. Cancers driven by BRCA mutations have a specific vulnerability: because their DNA repair system is already broken, they are unusually sensitive to drugs that block a backup repair pathway. These drugs, called PARP inhibitors, cause cancer cells to accumulate so much DNA damage that they die, while leaving normal cells (which still have working BRCA proteins) largely unharmed. This concept, known as synthetic lethality, has proven effective in clinical trials, with PARP inhibitors outperforming standard chemotherapy in BRCA-mutant breast cancer.
For people who carry a high-risk mutation but have not been diagnosed, preventive options include more frequent and earlier screening (often starting in the mid-20s for BRCA carriers), risk-reducing medications, and in some cases preventive surgery. The specific approach depends on which gene is involved, your age, and your personal and family cancer history.
Who Should Consider Genetic Testing
Genetic testing for breast cancer genes is most informative when there are specific red flags in your personal or family history. These include breast cancer diagnosed before age 50, ovarian cancer at any age, breast cancer in both breasts, male breast cancer in the family, multiple relatives with breast or ovarian cancer on the same side of the family, or Ashkenazi Jewish ancestry combined with any breast cancer history.
Testing typically involves a blood or saliva sample analyzed through a multigene panel that screens for BRCA1, BRCA2, PALB2, ATM, CHEK2, TP53, PTEN, and other relevant genes simultaneously. Results fall into three categories: positive (a known harmful mutation was found), negative (no mutation detected), or a variant of uncertain significance, which means a change was found but its impact on cancer risk is not yet clear. A genetic counselor can help interpret results in the context of your specific family history and guide next steps.