By Helen Briggs
Health reporter, BBC News
A radiologist studies mammograms
Five genetic clues to why some women have a family history of breast cancer have been identified by UK researchers.
It brings to 18 the number of common genetic variations linked to a small increased risk of breast cancer.
The Cambridge University-led research, published in Nature Genetics, could see targeted screening and treatment of women more likely to get breast cancer.
It is thought about one in 20 of all breast cancers are down to inherited faults in known genes.
Breast cancer is the most common cancer in the UK with more than 45,500 new cases diagnosed each year.
The precise reasons why a woman develops the disease are still unknown.
However, inherited, environmental and lifestyle factors are all thought to play a role.
In the largest project of its kind and funded by Cancer Research UK, the researchers scanned the entire genetic code of around 4,000 British patients with a family history of breast cancer.
They then studied the DNA of another 24,000 women, with and without breast cancer.
The researchers found five "spots" on the human genome linked to a family history of breast cancer. Another 13 have already been located.
Scientists also know about two high risk genes which are more likely to be defective in someone with breast cancer, known as BRCA1 and BRCA2.
Dr Douglas Easton of the University of Cambridge, is lead author of the study.
He told the BBC: "We know for sure that these gene variations are associated with risk.
Breast cancer risk factors
Breast cancer risk factors are related to lifestyle, life choices and the environment, or inherited
Established risk factors include age, alcohol, genetics, HRT, pregnancy, the contraceptive pill and weight
Possible risk factors include diet and stress
Source: Breakthrough breast cancer
"It is not the whole picture but it will contribute ultimately to genetic profiling of risk.
"It also contributes to our understanding of why the disease develops and will lead to a better understanding of the biology of the disease."
Women with a strong family history of breast cancer are already given early screening for signs of tumours.
They are also entitled to genetic tests if they have a close relative with breast cancer.
The 18 genetic changes linked to breast cancer are not currently tested for. They are thought to account for around 8% of inherited cases of breast cancer.
Dr Caroline Hacker, policy manager at Breakthrough Breast Cancer, said: "This could lead to new genetic tests which may help identify women who have an increased risk of breast cancer due to inherited faults in genes.
"Hereditary breast cancer is rare and only around one in 20 of all breast cancers are due to inherited faults in breast cancer genes.
"Although there isn't anything we can do about the genes we inherit, we do know that you can reduce your risk of breast cancer by maintaining a healthy weight, limiting alcohol consumption and exercising regularly."
Dr Helen George, head of science information at Cancer Research UK, said: "This research takes us a step closer to developing a powerful genetic test for the disease.
"Such a test could help doctors identify women who have an increased breast cancer risk so that they can make informed decisions about how to take steps to reduce their chance of developing the disease."