Aromatase: An enzyme involved in the production of estrogen that acts by catalyzing the conversion of testosterone (an androgen) to estradiol (an estrogen). Aromatase is located in estrogen-producing cells in the adrenal glands, ovaries, placenta, testicles, adipose (fat) tissue, and brain.
The growth of some breast cancers is promoted by estrogens. For example, the drug letrozole (brand name: Femara) is an antiestrogen sometimes used to treat such estrogen-dependent tumors. The drug acts by inhibiting the enzyme aromatase which lowers the level of the estrogen, estradiol.