Men with a family history of prostate or testicular cancer are being urged to take part in a national genetic study.
Scientists at the Institute of Cancer Research are searching for the genes that give some men an increased risk of the diseases.
They need to screen hundreds more patients to understand why it runs in families.
The national call-up, launched in London at the start of the sixth Everyman Male Cancer Awareness Month, is expected to lead to new treatments and better diagnosis.
It could also lead to a national screening programme for prostate cancer, now the commonest cancer in men.
The benefit of finding these genes is to find people who really are at increased risk so that we could target the screening more effectively
Prostate cancer is kills one man every hour in the UK while testicular cancer incidence has risen by 70% in the past two decades.
Finding the genes that give some men a higher risk would make a big difference, said Professor Colin Cooper of The Institute of Cancer Research in Sutton, Surrey.
It would enable men to find out whether they definitely had the gene and thus an increased risk of developing the disease, he said.
"The only way that's going to happen is through further research to identify the genes that predispose to prostate and testicular cancer," he told BBC News Online.
Genes are implicated in about 10-15% of prostate cancer cases. Testicular cancer can also run in families - a man with a brother who has testicular cancer has an eight to ten fold increased risk of getting it himself.
Scientists have already found some genetic clues as to why this is the case. As many as six mutated genes are implicated in prostate cancer and at least one in testicular cancer.
They now need to study more families with histories of both diseases to narrow down their search.
Oncologist Dr Ros Eeles said finding the prostate cancer genes could lead to a national screening programme in 10 to 15 years.
"At the moment if you asked me whether I would provide a prostate cancer screening programme for the whole country, yes it could be done but we would be screening a lot of men unnecessarily.
"The benefit of finding these genes is to find people who really are at increased risk so that we could target the screening more effectively."
The institute is looking for families with three or more cases of prostate cancer diagnosed at less than 70 years of age and families with two or more cases of testicular cancer.
Prospective volunteers are asked to contact their specialist or GP or phone the institute on 0208 661 3667 (prostate cancer) or 0208 722 4053 (testicular cancer).
Terry Palmer, aged 73, was diagnosed with prostate cancer in 1999. He went to his doctor early and was successfully treated because his brother had already suffered from the disease.
"If they get it early, there's a lot that can be done," he said. "One wants to encourage everybody, if there's the slightest doubt, go and have it checked out."
The cancer charity, Macmillan Cancer Relief, has given a cautious welcome to the study.
"Macmillan Cancer Relief believes that genetic screening, where linked to medical care and preventative advice, may enable people to take control of their own health and make decisions about their future cancer care," said Chief Executive Peter Cardy.
"However, Macmillan believes that such screening be used only with the right counselling, advising people on the accuracy of the tests and the likely implications of both a positive and negative result."