Hormone receptor expression remains the most important biomarker in breast cancer and is the cornerstone of optimal individual patient treatment decisions. Breast cancer cells may have one, both, or none of these receptors. Estrogen receptors (ER) and progesterone receptors (PR; also called PgR) may be found in breast cancer cells. Cancer cells with these receptors depend on estrogen and related hormones, such as progesterone, to grow.ER-positive: Breast cancers that have estrogen receptors are called ER-positive (or ER+) cancers. PR-positive: Breast cancers with progesterone receptors are called PR-positive (or PR+) cancers.