The sexes in humans and other animals typically display numerous differences. This belies the fact that the two major hormone classes responsible for these differences, androgens and estrogens, differ only subtly in their chemical backbones. Androgens have a six-carbon nonaromatic ring—the “A ring” (Fig. 1)—in their steroid skeleton, whereas estrogens have an aromatic A ring. Remarkably, a single protein, steroid aromatase (also called estrogen synthase), is the only known enzyme capable of “aromatizing” the A ring in androgens such as testosterone and androstenedione to produce estrogens such as estradiol and estrone.
