17β-Hydroxysteroid dehydrogenase type 1 (17HSD1) catalyzes the reaction from low-activity estrone to high-activity estradiol and is thus a temping target for the inhibition of estradiol biosynthesis. Among the several 17HSDs 17 HSD1 is the one essential for estradiol biosynthesis in developing follicles in the ovaries. The type 1 enzyme is also expressed in a notable portion of breast tumors where it may be responsible for maintaining a high intracellular estradiol concentration. Therefore, 17HSD1 inhibitors may be useful in the treatment and/or prevention of estrogen-dependent malignancies, such as breast cancer. Detailed knowledge of the function and structure of the 17HSD1 enzyme significantly facilitates the development of specific inhibitors for it.