DHRS9, also known as dehydrogenase/reductase SDR family member 9, functions as a homotetramer that converts both 3-alpha-tetrahydroprogesterone (allopregnanolone) and 3-alpha-androstanediol to dihydroxyprogesterone and is thought to play a role in retinoic acid biosynthesis. Recombinant human DHRS9 protein, fused to His-tag at N-terminus, was expressed in E.coli and purified by using conventional chromatography.