Although breast cancer is a disease we usually associate with women, around 350-400 new cases of male breast cancer are diagnosed in the UK each year.
Now, a study has found men with naturally high levels of the female hormone oestrogen are at greater risk of the disease than other men.
Researchers found that men with the highest levels were two and a half times more likely to develop the disease than those with the lowest.
This is the first time oestrogen circulating in the blood has been positively linked to male breast cancer, despite the known connection between the hormone and breast, womb and ovarian cancers in women.
Each year, almost 50,000 women in the UK are diagnosed with breast cancer, making it primarily - but not exclusively - a female disease.
The new study compared oestrogen levels in 101 men who went on to develop breast cancer and 217 healthy men.
Lead researcher Professor Tim Key, Cancer Research UK's hormone and nutrition expert at Oxford University, said: "We've shown for the first time that just like some forms of the cancer in women, oestrogen has a big role to play in male breast cancer.
"So now the challenge is to find out exactly what this hormone is doing to trigger this rare form of the disease in men, and why some men have higher levels of oestrogen in their blood. Our discovery is a crucial step forward in understanding the factors behind male breast cancer."
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The symptoms of breast cancer and the way it is diagnosed and treated are very similar in both men and women.
For men, age is the biggest risk factor with almost eight out of 10 cases being diagnosed in patients at least 60 years old.
Dr Julie Sharp, head of health information at Cancer Research UK, said: "Breast cancer in men isn't discussed very often, so a diagnosis can be a big shock for the small group of men who develop the disease.
"Some of the oestrogen variation in men will simply be natural, but for others there may be a link to being overweight.
"Fat cells in the body are thought to drive up the body's level of this hormone in men and women, so this is another good reason to try and keep a healthy weight.
"This early research is crucial in understanding why these men get breast cancer, so that one day we can treat it more effectively."
The research is reported in the Journal of Clinical Oncology.