Hormones are substances that function as chemical messengers in the body. They affect the actions of cells and tissues at various locations in the body, often reaching their targets through the bloodstream.
The hormones estrogen and progesterone are produced by the ovaries in premenopausal women and by some other tissues, including fat and skin, in both premenopausal and postmenopausal women and in men. Estrogen promotes the development and maintenance of female sex characteristics and the growth of long bones. Progesterone plays a role in the menstrual cycle and pregnancy.
Estrogen and progesterone also promote the growth of some breast cancers, which are called hormone-sensitive (or hormone-dependent) breast cancers. Hormone-sensitive breast cancer cells contain proteins called hormone receptors (estrogen receptors, or ERs, and progesterone receptors, or PRs) that become activated when hormones bind to them. The activated receptors cause changes in the expression of specific genes, which can stimulate cell growth.
To determine whether breast cancer cells contain hormone receptors, doctors test samples of tumor tissue that have been removed by surgery. If the tumor cells contain estrogen receptors, the cancer is called estrogen receptor positive (ER positive), estrogen sensitive, or estrogen responsive. Similarly, if the tumor cells contain progesterone receptors, the cancer is called progesterone receptor positive (PR or PgR positive). Breast tumors that contain estrogen and/or progesterone receptors are sometimes called hormone receptor positive (HR positive). Most ER-positive breast cancers are also PR positive.
Breast cancers that lack ERs are called ER negative, and if they lack both ER and PR they may be called HR negative.
Approximately 67%–80% of breast cancers in women are ER positive (1, 2). Approximately 90% of breast cancers in men are ER positive and approximately 80% are PR positive (3).