This is an uncontrolled growth of cells within the breast ducts.

As the baby begins to suck, the milk is "let down" from the breast ducts into the nipple.

A mammogram may also find ductal carcinoma in situ, abnormalcells in the lining of a breast duct, which may become invasive cancer in some women.

DCIS is, by definition, confined to the breast ducts.

The abnormal cells, which are encased in breast ducts, are removed before they develop into invasive cancer.

Says Dr. McHugh: "If the cancer breaks through the wall of the breast duct, it's considered invasive.

A very thin, flexible, lighted tube attached to a camera is inserted through the nipple, and threaded into the breast ducts deep in the breast.

Intraductal carcinoma is a noninvasive condition in which abnormal cells are found in the lining of a breast duct.

Most breast cancers, though, begin in the breast ducts or tubes that connect the lobules to the nipple.

The breast duct and lobules, where breast cancer begins, contain fluid with estrogen levels many times higher than the levels in the blood.