It is a chemical (hormone) responsible for estrous (female sexual) characteristics in female. The three major naturally occurring estrogens in women are estrone (E1), estradiol (E2), and estriol (E3). E2 is the predominant estrogen during reproductive years, both in terms of absolute serum levels as well as in terms of estrogenic activity. Estradiol is responsible for the regulation of the estrous and menstrual female reproductive cycles and for the development and maintenance of female secondary sex characteristics. Estradiol plays a key role in germ cell maturation and numerous other, non−gender-specific processes, including growth, bone metabolism, nervous system maturation, and endothelial responsiveness. Estrogens are crucial for the normal development and maintenance of the breasts and the uterus. Measurement of serum E2 is a blood test which serves an integral role in the assessment of reproductive function in females and in the assessment of infertility, oligomenorrhea, and menopausal status. E2 is commonly measured for monitoring ovulation induction, as well as during preparation for in vitro fertilization.