Breasts are made up of fibrous tissue (or connective tissue), glandular tissue (the type of tissue that produces milk), and fatty tissue. Breast cancer is a cancer of the breast tissue. It arises from the uncontrollable cell growth of cells in the breast.

Breast cancer can be categorised in several ways. Most often it is classified by where it originates and whether it moves from that spot.

For prediction of patient prognosis and treatment plan, breast cancer has been clinically staged based on:

• Tumour size (T)
• Lymph node status (N)
• Metastasis (M)

Breast cancer is also divided into different molecular subtypes according to the biological characteristics of tumour cells, including:

• Estrogen Receptor (ER)
• Progesterone Receptor (PR)
• Human Epidermal Growth Factor Receptor Type II (HER-2)

Biological characteristics of tumour cells have also been included in medical evaluation to select targeted therapies.