HSD3B1 (3-beta hydroxysteroid dehydrogenase) is a bifunctional enzyme that catalyzes the oxidative conversion of Delta(5)-ene-3-beta-hydroxy steroid and the oxidative conversion of ketosteroids. The 3-beta-HSD enzymatic system plays a crucial role in the biosynthesis of all classes of hormonal steroids.There are two HSD3B1 proteins, type I and type II. Expressed by different genes, they function in different areas of the body. HSD3B1 has been shown to be a highly specific and sensitive trophoblast-associated marker.