Breast cancer is a cancer that starts in the tissues of the breast.

There are two main types of breast cancer:

• Ductal carcinoma starts in the tubes (ducts) that move milk from the breast to the nipple. Most breast cancers are of this type.
• Lobular carcinoma starts in parts of the breast, called lobules, that produce milk.

In rare cases, breast cancer can start in other areas of the breast.

Breast cancer may be invasive or noninvasive. Invasive means it has spread to other tissues. Noninvasive means it has not yet spread. Noninvasive breast cancer is referred to as "in situ."

• Ductal carcinoma in situ (DCIS), or intraductal carcinoma, is breast cancer in the lining of the milk ducts that has not yet invaded nearby tissues. It may progress to invasive cancer if untreated.
• Lobular carcinoma in situ (LCIS) is a marker for an increased risk of invasive cancer in the same or both breasts.

Many breast cancers are sensitive to the hormone estrogen. This means that estrogen causes the breast cancer tumor to grow. Such cancers have estrogen receptors on the surface of their cells. They are called estrogen receptor-positive cancer or ER-positive cancer.

Some women have what's called HER2-positive breast cancer. HER2 refers to a gene that helps cells grow, divide, and repair themselves. When cells have too many copies of this gene, cells -- including cancer cells -- grow faster. Experts think that women with HER2-positive breast cancer have a more aggressive disease and a higher risk of recurrence than those who do not have this type.