Breast cancer is a malignant tumor that forms from the uncontrolled growth of abnormal breast cells. Malignant tumors can invade and destroy surrounding tissue and spread to other parts of the body. Breast cancer usually affects tissues involved in milk production (ductal and lobular tissues). The cause of most breast cancers is unknown; however, a small percentage of breast cancers tend to cluster in families. These cancers can be caused by mutations in particular genes, such as BRCA1 or BRCA2. In some cases, genetic syndromes involving other cancers also include an increased risk of breast cancer.

Early breast cancer usually does not cause pain and may exhibit no noticeable symptoms. As the cancer progresses, signs and symptoms can include a lump or thickening in or near the breast; a change in the size or shape of the breast; nipple discharge, tenderness, or retraction (turning inward); and skin irritation, dimpling, or scaliness. These changes can also be caused by conditions other than breast cancer.

Hereditary breast cancers, which are those caused by inherited gene mutations, tend to occur earlier in life than noninherited (sporadic) cases and are more likely to involve both breasts.

Each year, more than 211,000 cases of invasive breast cancer are diagnosed in women in the United States. Most breast cancers occur in women, but they can also develop in men. More than 1,600 cases of male breast cancer are diagnosed each year.

An estimated 5 percent to 10 percent of all breast cancers are hereditary. Some mutations in particular genes associated with breast cancer are more common among certain geographic or ethnic groups, such as people of Ashkenazi (central or eastern European) Jewish heritage and people of Norwegian, Icelandic, or Dutch ancestry.

Variations of the ATM, BRCA1, BRCA2, CHEK2, and RAD51 genes increase the risk of developing breast cancer.

The AR, DIRAS3, ERBB2, and TP53 genes are associated with breast cancer.

These genes provide instructions for making proteins that help regulate the growth and division of cells. Some of these proteins are involved in detecting and repairing damaged DNA. Others help the cell respond to external signals, such as hormones and growth factors. When a gene mutation alters the structure or function of these proteins, cells in the breast can grow and divide uncontrollably and form a tumor.

BRCA1 and BRCA2 are major genes related to hereditary breast cancer. People who have inherited certain mutations in these genes have a high risk of developing breast cancer, ovarian cancer, and several other types of cancer over their lifetimes. Men with BRCA1 mutations have an increased risk of developing prostate cancer. Mutations in the BRCA2 gene are associated with an increased chance of developing male breast cancer and cancers of the prostate, pancreas, gallbladder, bile duct, and gastrointestinal tract. Melanoma (an aggressive form of skin cancer) and lymphoma (a cancer of immune system cells) also are more common among people who have BRCA2 mutations.

Inherited changes in several other genes, including ATM, CHEK2, and RAD51, have been associated with an increased risk of breast cancer. Although some research suggests that certain versions of the AR gene also may be associated with breast cancer risk, several other studies have not shown this connection.

Other gene mutations are acquired during a person's lifetime and are present only in certain cells. These changes, which are called somatic mutations, are not inherited. Somatic mutations in the ERBB2 (also called Her-2/neu), DIRAS3 (formerly ARHI), and TP53 genes have been identified in some cases of breast cancer.

Additional genetic changes are likely associated with breast cancer risk; however, many of these genes have not been identified or characterized.

The way that breast cancer risk is inherited depends on the gene involved. For example, mutations in the BRCA1 and BRCA2 genes are inherited in an autosomal dominant pattern, which means one copy of the altered gene is sufficient to increase the chance of developing cancer. In other cases, the inheritance of breast cancer risk is unclear. It is important to note that people inherit an increased risk of cancer, not the disease itself. Not all people who inherit mutations in these genes will develop cancer.

Somatic mutations (genetic changes that occur only in breast cancer cells) occur during a person's lifetime and are not inherited.