STX64 is a potent, irreversible inhibitor of steroid sulfatase. Inhibition of steroid sulfatase (STS), the enzyme responsible for the hydrolysis of steroid sulfates, represents a potential novel treatment for postmenopausal women with hormone-dependent breast cancer. Estrone and DHEA are formed by this sulfatase pathway and can be converted to steroids (estradiol and androstenediol, respectively), which have potent estrogenic properties. STX64 (667 coumate) is a potent tricylic coumarin-based sulfamate that irreversibly inhibits STS activity (IC50 = 8 nM in a placental microsomal assay system). Estrone sulfamate (EMATE) is also a potent STS inhibitor, but has estrogenic activity.