endoplasmic reticulum membrane
Aromatase (CYP19A1) catalyses the conversion of androstenedione (ANDST) to estrone (E1). Defects in CYP19A1 can cause aromatase excess syndrome (AEXS; MIM:139300) and aromatase deficiency (AROD; MIM:613546). Affected individuals cannot synthesise endogenous estrogens. In females the lack of estrogen leads to pseudohermaphroditism and progressive virilization at puberty, whereas in males pubertal development is normal. Mutations causing AEXS include C437Y, R375C, R365Q and E210K (Ito et al. 1993, Morishima et al. 1995, Carani et al. 1997, Maffei et al. 2004).