Estrone is a HORMONE that is part of the estrane group of steroid hormones. It is produced mainly in the ovaries and is the primary female sex HORMONE. It plays an important role in the development of secondary sexual characteristics in women, such as breast development and the regulation of the menstrual cycle. It is also involved in the formation of bone and fat tissue. In men, it is produced in small amounts by the testes and adrenal glands.