Estradiol is the most potent natural estrogen (the major female hormone). Estradiol is responsible for growth of the uterus, fallopian tubes, and vagina; breast development; maturation of the external genitals; deposition of body fat in a female distribution; and ending linear growth during adolescence. Estradiol also stimulates the proliferation of the uterine lining in the first half of the menstrual cycle. Estradiol is the most potent natural estrogen and circulates in plasma predominantly in a protein-bound form. In non-pregnant women, E2 is produced mainly by the ovaries. In men, the testes and androgen precursors are the principal source.