Scientists conducting the world's largest study of male breast cancer have identified a gene that raises the risk of developing the disease by half.
Findings from the new research suggest that the causes of the disease may differ between women and men.
Male breast cancer is a rare disease that tends to be forgotten but can be just as lethal as its female counterpart. Around 350 men in the UK are diagnosed with the disease each year, compared with 48,000 women.
It was already known that faulty BRCA2 genes are involved in around 10% of cases, a much higher proportion than among women. Changes in the RAD51B gene - which is involved in the repair of damaged DNA - also play a role, according to the new research. They increase the risk of male breast cancer by up to 50%.
Dr Nick Orr, from the Breakthrough Breast Cancer Research Centre at the Institute of Cancer Research in London, said: "This study represents a leap forward in our understanding of male breast cancer.
"It shows that while there are similarities with female breast cancer, the causes of the disease can work differently in men. This raises the possibility of different ways to treat the disease specifically for men."
An international team of scientists screened the genetic code of 823 male breast cancer patients and investigated 447,000 alterations in their DNA.
The study highlighted RAD51B, which when faulty also raises the risk of breast cancer in women. However, different parts of the gene are implicated in men's and women's cancers.
Anthony Swerdlow, co-leader of the Male Breast Cancer Study and Professor of Epidemiology at the Institute of Cancer Research, said: "Male breast cancer is rare, which makes it difficult to study. Through drawing on many hundreds of patients from this country and abroad, we can now start to unravel its causes.
"We will be continuing this research to try to find more genes that raise the risk of male breast cancer, in order to understand better the causes of this disease in men, and in women."
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Study participant Andrew Tokely, 47, a horticulturist from Capel St Mary, Suffolk, who learned he had male breast cancer in 2009, said: "When I was diagnosed with breast cancer I didn't even know men could get the disease.
Joining the Male Breast Cancer Study was great because it means I'm doing my bit to help find the causes. I'm glad to be a part of this research which I hope will lead to specific treatments for men in the future."
Dr Orr lead the research reported today in the journal Nature Genetics.