A simple saliva test that reveals a patient's risk of breast or prostate cancer may soon be readily available, a study suggests.
Research involving more than 1,000 experts has found genes that help determine from birth whether or not somebody will develop cancer during their life.
Screening patients for genetic variants linked to cancer could help doctors predict more accurately whether they are at high risk of the disease, and in some cases begin treatment to stop it developing.
The tests could identify the one in 100 men whose genes give them a 50pc chance of developing prostate cancer, and the one in 100 women whose likelihood of developing breast cancer is 30pc.
These tests could be carried out by GPs, would be cheaper than breast screening methods and may help spare patients at low risk of cancer from additional, more invasive examinations.
Dr Harpal Kumar, chief executive of Cancer Research UK, said the research "can help us identify exactly what is driving different types of cancer, which could enable us to develop new therapeutic approaches".
The researchers studied the DNA of more than 100,000 cancer patients and compared it against the general population, focusing on 200,000 particular genetic variants.
Their analysis revealed more than 80 sections of genetic code that contribute to the risk of developing cancers, including 49 for breast cancer, 23 for prostate cancer and 11 for ovarian cancer.
Although about 60pc of the genetic risk factors for all three cancer types remain unexplained, the findings are a significant step forward in our understanding of the inherited causes of the condition.
In the case of prostate cancer, 18 of the 23 variants applied specifically to the most aggressive types of the disease.
Prof Ros Eeles, of the Institute of Cancer Research in Britain, said: "These results are the single biggest leap forward in finding the genetic causes of prostate cancer yet made. They allow us, for the first time, to identify men who have a very high risk of developing prostate cancer."
Although the variants, known as single nucleotide polymorphisms (SNPs), make little difference on their own, when added together they can greatly raise the risk of the disease.
In prostate cancer, for example, the 1pc of men with the highest number of SNPs are 4.7 times more likely to develop the condition compared with the average man. Women in the top 1pc for breast cancer have a 30pc chance of developing the disease, excluding those who carry the BRCA1 and BRCA2 genes, which are already known to cause a high risk of the disease.