Estrogen is a key player in the biology of breast cancer. Estrogen links with the estrogen receptor protein to influence breast tissue function. Estrogen receptor (ER) can be measured in the breast cancer tissue. It is one of the key bio-markers that is tested to help determine your course of treatment. Progesterone receptor (PR) is also an important test to perform on any breast cancer. A cancer that measures positive for either estrogen or progesterone or both receptors is considered hormone-receptor positive. Hormone receptors are like antennae on a cell. Estrogen sends signals through the receptors to tell breast cancer to grow. After a breast cancer is removed, the cancer cells are tested for estrogen and progesterone receptors. The more estrogen and progesterone receptors that are present, the more likely the cancer will respond to hormonal therapy. If the tumor does not have estrogen or progesterone receptors, antiestrogen therapy will not be helpful.

What this means is that the oncologist is looking to see if the tumor cells are likely to respond to anti-estrogen treatment. In general, tumors in postmenopausal women are more likely to be estrogen-receptor positive and tumors in premenopausal women estrogen-receptor negative. Knowing the receptor status helps to determine whether hormone replacement therapy is an option.

The S-phase lab test reveals the number of cells dividing in the tumor, the S-phase fraction usually showing how quickly a cell is dividing. A low S-phase fraction is an indication that the tumor is slow growing and a high S-phase fraction shows that the cells are rapidly dividing and that the tumor is more aggressive. This test is not available everywhere and gives the same information as nuclear grade, but is more reliable.