The steroid hormones and their receptors are involved in the regulation of eukaryotic gene expression and affect cellular proliferation and differentiation in target tissues.

Estrogen receptors (ER) are a group of nuclear hormone receptors activated by the hormone estrogen. ER is found in normal epithelial cells of the breast and endometrium, as well as in breast cancer cells. Dysfunction in Estrogen and the Estrogen Receptor has been closely linked to obesity, breast cancer, ovarian cancer, colon cancer, prostate cancer, and endometrial cancer.

Breast tumors that are ER positive may also show a more favorable response to anti-estrogen therapies. Estrogen receptor (ER) expression is the main indicator of potential responses to endocrine therapy (ET), and approximately 70% of human breast cancers (BCs) are hormone-dependent and ER-positive.