Receptors are specialized proteins on the surface of or within cells that recognize and bind to other substances. The binding typically has a specific effect on the cells. Many, but not all, breast cancer cells have receptors that bind to the hormonesestrogen and progesterone. Breast cancer tumors with estrogen receptors (ER) and progesterone receptors (PR) depend on the hormones to grow and divide. ER and PR testing of breast tumor tissue determines if one or both types of receptors are present.

Knowing if a tumor depends on hormones to grow helps a health practitioner determine a person's risk of breast cancer recurrence and whether it can be treated with hormone therapy to block estrogen and progesterone. About two-thirds of breast cancer tissues are positive for both ER and PR.