Estrogen and progesterone receptor (ER/PR) tests identify whether hormones affect the way that breast cancer grows. The results of these tests help doctors decide whether to use hormone therapy to help stop the cancer from growing.

The hormones estrogen and progesterone attach to certain receptors on the breast cells to 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.