Steroid sulfatase/17β-HSD1-IN-3 (compound 19) is a dual inhibitor of steroid sulfatase ( STS ) and 17β-hydroxysteroid dehydrogenase type 1 ( 17β HSD1 ). Steroid sulfatase/17β-HSD1-IN-3 irreversibly inhibits hSTS activity with an IC 50 value of 27 nM. Steroid sulfatase/17β-HSD1-IN-3 can be used in the study of endometriosis and other estrogen-dependent diseases.