Estrogens (also oestrogens) are a group of steroid compounds, named for their importance in the oestrus cycle, and functioning as the primary female sex hormone. While estrogens are present in both men and women, they are usually present at significantly higher levels in women of reproductive age. They promote the development of female secondary sex characteristics, such as breasts, and are also involved in the thickening of the endometrium and other aspects of regulating the menstrual cycle. Like all steroid hormones, estrogens readily diffuse across the cell membrane; inside the cell, they interact with estrogen receptors. The three major naturally occurring estrogens in women are estradiol, estriol and estrone. Estrogens are used as part of some oral contraceptives and also in estrogen replacement therapy of post-menopausal women.