The abnormal development of cells lining, the lobules or ducts of the breast, causes breast cancer. Breast cancer is caused by a single altered cell or a clone. It usually takes a long time for such clonal cells to become fully malignant, invade, and spread. During the early stages of breast cancer, it is normally painless, although growths can stretch the skin and breast tissue, causing discomfort or pain. The pain may occur anywhere in the breast or in the nipple.

There are many defined forms of breast cancer based on their invasiveness compared to the initial tumor locations among the wide group of distinct breast carcinomas. It's crucial to understand the differences between the subtypes because they have varying prognoses and treatment options. At the molecular level, there are striking similarities between normal development and breast cancer progression, leading to the theory that breast cancer is produced from mammary cancer stem cells. Several signaling pathways, including estrogen receptors (ERs), HER2, and the Wnt/-catenin signaling pathways, influence normal breast growth and mammary stem cell proliferation.

Breast cancer is linked to a number of risk factors, including age, hormone status, family history, and genetic susceptibility. Patients with first-degree female relatives who have been diagnosed with breast cancer are more likely to develop the disease. Breast cancer treatment can be complicated and depends on a number of factors. Breast cancer treatment can be classified as either extensive (mastectomy) or conservative (breast preservation) (lumpectomy).

Adjuvant or systemic therapy is used to treat persons with breast cancer in addition to local surgical therapy. Chemotherapy or hormone therapy are examples of adjuvant therapy. Fortunately, advances in diagnostic and therapeutic capacities have resulted in a higher survival rate among those diagnosed with breast cancer.