Google this, I found this: Exemestane is an oral steroidal aromatase inhibitor that is used in ER-positive breast cancer in addition to surgery and/or radiation in post-menopausal women. ... Type II aromatase inhibitors such as anastrozole and letrozole, by contrast, are not steroids and work by interfering with the aromatase's heme. There's a lot of info out there on this.