Both reductive forms of 17 beta-hydroxysteroid dehydrogenase (types 1 and 3) are expressed during development in the mouse testis.

Androstenedione is reduced to form testosterone by 17-beta-hydroxysteroid dehydrogenase (17 beta HSD) and two different reductive isoforms of the enzyme have been identified (types 1 and 3). In this study, levels of mRNA encoding both reductive isoforms have been measured during fetal and post-natal development in the mouse. In fetal and neonatal testes mRNA encoding both type 1 and type 3 isoforms was present at relatively high levels reaching a peak at postnatal day 5. Thereafter, mRNA levels of both 17 beta HSD isoforms fell to low levels until day 30 when there was a marked increase in the levels of the type 3 isoform. The presence of the type 1 17 beta HSD enzyme in fetal testes may explain the virilization of the mesonephric (Wolffian) duct which occurs in pseudohermaphrodite individuals lacking the type 3 isoform.