Scientists have discovered why some families are affected by many different types of cancer, thanks to genetic testing.
In tests on 1,100 patients affected by a rare cancer called sarcoma, more than half were born with gene mutations known to increase cancer risk.
The study, published in The Lancet Oncology, said the inherited mutations could become targets for treatments.
And families affected by cancer could be offered screening and advice.
Inherited mutations in genes linked to breast, ovarian and bowel cancer, among others, were found to be common in sarcoma patients.
The researchers, from The Institute of Cancer Research, The Royal Marsden NHS Foundation Trust and University College London Hospitals NHS Foundation Trust, said the results were revealing and gave a more detailed picture of how cancer risk is inherited.
Prof Ian Judson, who led the study at The Institute of Cancer Research in London, said: "We are going to need to think differently about inheritance.
"Sometimes you are confident there is something going on in the family - but standard patterns of risk might not fit.
"Now we can work out why that is because there may be two or three mutations going on."
Sarcoma is a very unusual cancer that develops in the bones, muscles or soft tissues of the body. There are around 3,000 cases of this cancer in the UK each year.
Because it can often affect children and young people, with low survival rates, the UK research team decided to find out more about the genetic causes of this particular cancer.
They analysed the DNA sequence of 72 genes linked to increased cancer risk in 1,162 patients with sarcoma.
One in five patients was found to have mutations or errors in more than one of the cancer genes tested.
Patients with mutations in multiple cancer genes were found to be more likely to develop cancer at a younger age than those with just one single genetic mutation.
So this suggests that many genetic mutations working together could be causing sarcoma in some patients, rather than just a single gene error causing their disease.
Prof Judson said that when such mutations are found, families could now be offered genetic screening and given proper advice on treatment.
Sarah McDonald, director of research at Sarcoma UK, said: "If we can identify individuals at high risk of developing sarcomas this could lead to earlier detection and more effective treatment of these tumours."
BBC Health News