Breast cancers can be sorted according to various characteristics and for a variety of reasons. In the early days - last century - when treatment options were limited, breast cancers were classified according to what they looked like under the microscope. It was very primitive but effective in distinguishing one type from another.

But once breast cancers could be studied more closely, even genetically, other more subtle characteristics were identified. Many of these more subtle differences could then be used to help tailor treatment.

In the middle of the twentieth century, receptors (proteins) were found on the surfaces of some breast cancer cells that were clues to cancer growth and behavior. Estrogen and progesterone receptors were the first such receptors identified. Identification of these first receptors allowed pharmaceutical companies to develop drugs that cold block the receptors and inhibit the growth of the cancer. Tamoxifen, a drug that binds to estrogen receptors and blocks their function, was used as the first targeted therapy for breast cancer. Other drugs, similar to tamoxifen but with fewer side effects, have since been developed (e.g., arimidex.)

About fifteen years ago, another receptor was identified: Her-2 neu. Shortly thereafter, a drug (herceptin) was developed that targets that receptor and it, too, has been helpful in improving overall survival.

Because these receptors provide so much useful information about the behavior of all varieties of breast cancer, researchers have recently decided to re-categorize all breast cancers into four types, depending on the degree to which an individual patient's breast cancer cells express any of these receptors. These four types are now known as Luminal A, Luminal B, Her 2, and triple negative.

1. Type A breast cancers express estrogen or progesterone receptors, but not Her 2.

2. Type B breast cancers express estrogen or progesterone receptors, and they also express Her 2

3. Her 2 breast cancers only express Her 2, they do not express either estrogen or progesterone receptors.

4. Triple negative breast cancers do not express any of the receptors.

The reason scientists decided to group breast cancers into these four subtypes is because each one tends to respond differently to different therapies - with those expressing estrogen and progesterone receptors responding well to drugs like tamoxifen, and Her 2 types responding well to herceptin. Her 2 cancers also respond well to chemotherapy.

Sadly, triple negative cancers respond well (actually, very well) to chemotherapy, but not to drugs like tamoxifen or herceptin. Unfortunately, patients with triple-negative breast cancer tend to have a good initial response to chemotherapy but then go on to experience recurrence and death.