Genes called BRCA1, BRCA2, TP53, PALB2 or PTEN are the main ones linked to a high risk of breast cancer. Everyone has these genes. If there's a fault in one of them, the risk of breast cancer is higher.
Faulty BRCA1 and BRCA2 genes also increase the risk of ovarian cancer. A blood test can identify whether you have a fault in one of these high-risk genes.
Having a faulty high-risk gene linked to breast cancer isn't common, says Professor Gareth Evans, consultant medical geneticist at St Mary's Hospital in Manchester. "Mutations in the high-risk genes affect 1 in 400 people or less," he says.
What is the normal risk of breast cancer?
In the UK, a woman's lifetime risk of developing breast cancer is 10-12%. Out of every 100 women, around 10-11 will develop breast cancer by the time they are 80 years old. It can affect anyone. You can get it even if you don't have a faulty gene.
If more than one person in a family has breast cancer, this doesn't mean the faulty genes are present. It could be chance, as breast cancer is common.
What's the risk if you have a faulty gene?
If you have a faulty gene, it doesn't mean you'll definitely develop breast cancer, but you are at higher risk.
Having a fault in one of the high-risk breast cancer genes raises the risk of developing breast cancer to between 40% and 85%.
Out of every 100 women with a faulty gene, between 40 and 85 of them will develop breast cancer in their lifetime.
Who is tested for faulty genes?
If you are diagnosed with breast cancer and have a strong family history of breast and/or ovarian cancer, you will usually be offered a blood test to see whether you have a faulty gene.
If the test is positive, this means you have a higher risk of developing cancer in your other breast and, with BRCA1 and BRCA2, developing ovarian cancer.
"Women who have a mother or sister who has tested positive can opt for a simple genetic test to find out whether they've got the faulty gene," says Professor Evans.
If you don't have a relative who has tested positive for a faulty gene, family history needs to be strong to justify testing.
"Looking at family history means looking at the combinations of breast and ovarian cancer, and young-onset [younger than 50] breast cancer in the family," says Professor Evans.
"For example, a family history of three women in their 30s or four women in their 40s having cancer would easily qualify someone for genetic testing."
What if a woman without cancer has a faulty gene?
If you have a faulty gene, you have a high lifetime risk of breast and ovarian cancer.
Depending on your age and estimated risk, you'll be offered mammograms or magnetic resonance imaging (MRI) scans, or both.
You can still follow general health advice, too. "Women can reduce their risk by not putting on weight, by eating a good diet and by taking regular exercise," says Professor Evans.
"The problem is that they probably can't reduce their risk to much below 50%. That's why a high proportion of those women opt for risk-reducing surgery."
This involves surgical removal of the breasts (mastectomy) and sometimes removal of the ovaries (oophorectomy) to reduce the risk of cancer developing.
These are major operations, and you and your medical team need to discuss the risks and implications before any decisions are made.
"If a woman has a double mastectomy [removal of both breasts], her risk of developing breast cancer is reduced by 90-95%," says Professor Evans.
Where can I get help if I'm worried about my risk?
If two or three women in your family have had breast cancer and you're worried about your risk, see your GP.
If your GP thinks you have an increased risk, you can be referred to a local breast clinic. If there's already been a positive gene test in the family, they can refer you to a genetic clinic.
Read more about genetic tests for cancer risk genes.