Although certain risk factors may increase a man's chances of developing breast cancer, the cause of most breast cancers in men is unknown.

Hormone levels

Breast cells normally grow and divide in response to female hormones such as estrogen. The more cells divide, the more chances there are for mistakes to be made when they are copying their DNA. These DNA changes can eventually lead to cancer (see below).

Factors that change the ratio of female and male hormones in the body can therefore have an effect on breast cancer risk. Many of these were described in the section, "What are the risk factors for breast cancer?"

Gene changes (mutations)

Researchers are making great progress in understanding how certain changes in DNA can cause normal cells to become cancerous. DNA is the chemical in each of our cells that makes up our genes -- the instructions for how our cells function. We usually resemble our parents because they are the source of our DNA. However, DNA affects more than how we look.

Some genes contain instructions for controlling when our cells grow, divide, and die. Certain genes that speed up cell division are called oncogenes. Others that slow down cell division or cause cells to die at the appropriate time are called tumor suppressor genes. Cancers can be caused by DNA mutations (defects) that turn on oncogenes or turn off tumor suppressor genes.

Acquired gene mutations

Most DNA mutations related to male breast cancer occur during life rather than having been inherited before birth. It's not clear what causes most of these mutations. Radiation to the breast area is a factor in a small number of cases. Some acquired mutations of oncogenes and/or tumor suppressor genes may result from cancer-causing chemicals in our environment or diet, but so far studies have not identified any chemicals that are responsible for these mutations in male breast cancers.

Inherited gene mutations

Certain inherited DNA changes can cause a high risk of developing certain cancers and are responsible for cancers that run in some families.

Some breast cancers are linked to inherited mutations of the BRCA1 or BRCA2 tumor suppressor genes. Normally, these genes make proteins that help cells recognize and/or repair DNA damage and prevent them from growing abnormally. But if a person has inherited a mutated gene from either parent, the chances of developing breast cancer are higher.

In women, mutations of BRCA1 and BRCA2 are responsible for about 5% to 10% of breast cancers. Women with either of these altered genes have a lifetime breast cancer risk of up to 80%.

In men, changes in the BRCA2 gene seem to be responsible for about 10% of breast cancer cases. The lifetime breast cancer risk for men with BRCA2 mutations is about 5% to 10%, which is much higher than for other men.

BRCA1 seems to play a role in only a small number of male breast cancers, but it may be more common in Jewish men. Recent studies suggest that BRCA1 mutations may increase the lifetime risk of breast cancer in men to about 1%.

Genetic testing

Genetic testing can be done to look for mutations in the BRCA1 and BRCA2 genes (or less commonly in other genes related to breast cancer). Testing may be helpful in some situations,but the pros and cons need to be considered carefully. For more information, see the section, "Can breast cancer in men be found early?"

Last Medical Review: 09/24/2008
Last Revised: 05/13/2009