a mixture of naturally occuring estrogens derived from pregnant mares' urine. It is composed of estrone, equilin, and 17-alpha-dihydroequilin along with other less prevalent estrogens. Estrogens enter cells of responsive tissues (female genital organs, breasts, hypothalamus, pituitary) and are transported into the nucleus. Inside the cell nucleus, estrogens stimulate specific RNA and protein synthesis. Estrogens also decrease the release of follicle-stimulating hormone (FSH), luteinizing hormone (LH), and luteotropic hormone due to an effect on the pituitary or hypothalamus. Estrogens are important in the development and maintenance of the female reproductive system and secondary sex characteristics.