a hormone found in both men and women, but at higher levels in women. Often called the female sex hormone, it is made mostly by the ovaries, and in smaller amounts in fat tissue. In girls, estrogen helps to regulate puberty, such as growth of breasts. In women, estrogen levels normally cycle on a monthly schedule to regulate menstruation and prepare the body for fertilization and reproduction. Estrogen may promote the growth of cancer cells in breast cancer. In men, estrogen is sometimes used to treat advanced prostate cancer by countering the action of testosterone. See also estrogen receptor assay, estrogen therapy, hormone, hormone therapy, ovary.