When a breast biopsy shows that cancer is present, other tests are used to determine which treatment will be the most effective.

Estrogen and progesterone receptor (ER/PR) tests identify whether hormones affect the way the cancer grows. The hormones estrogen and progesterone stimulate the growth of normal breast cells and some breast cancers. Breast cancer cells that lack these receptors (ER-negative and PR-negative cancers) are much less likely to respond to treatment with such medicines as tamoxifen or aromatase inhibitors. ER/PR-positive tumors have these receptors and are more likely to respond to treatment with tamoxifen or aromatase inhibitors.

The results of estrogen and progesterone receptor tests help your doctor decide whether to use hormone therapy to stop the cancer from growing.