Letrez 2.5^TM is a type II third generation aromatase inhibitor. Clinically it is used to treat postmenopausal women with either estrogen receptor positive or estrogen receptor unknown breast cancer. In women, tumors that contain estrogen receptors are classified as estrogen receptor-positive (ER+) tumors. Letrozole is known as a type II aromatase inhibitor, meaning, in simplest terms that it attaches to the aromatase enzyme and prevents it from converting androgens to estrogen. In slightly more complex terms, estrogens are produced by the conversion of androgens through the activity of the aromatase enzyme, and letrozole actually inhibits the production of estrogens in by competitive, (reversible) binding to the heme of the relevant cytochrome P450 unit.