Breast cancer is the most frequent malignant disease and the leading cause of cancer death among women in both economically developed and developing countries. Globally, 1.4 million new breast cancer cases are diagnosed each year, of whom approximately one-third die of the disease. The incidence rates are highest in the Western world, where the lifetime risk of developing breast cancer is estimated to be one in nine. Owing to increased awareness, early detection, and better treatment options available, breast cancer mortality rates have declined in recent years.

In the middle of the 19th century, the first reports emerged, describing familial aggregation of breast cancers. Today, positive family history is one of the most important risk factors for developing breast cancer. It is currently estimated that approximately 5–10% of all breast cancers have a hereditary background. These families show an apparently dominant inheritance pattern and are often characterized by an early age of onset, the overrepresentation of ovarian cancers, bilateral breast cancers, and male breast cancers.

Breast tumors express a number of immunohistochemical (IHC) markers providing both prognostic and predictive information. The estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor (EGFR) 2 (HER2) are among the most important IHC markers.