The AKR1D family of aldo-keto reductase includes 3-oxo-5-beta-steroid 4-dehydrogenase (EC 1.3.1.3) from Homo sapiens (AKR1D1), Rattus norvegicus (liver, AKR1D2), and Oryctolagus cuniculus (AKR1D3). 3-oxo-5-beta-steroid 4-dehydrogenase, also called delta(4)-3-ketosteroid 5-beta-reductase (EC 1.3.99.6), or delta(4)-3-oxosteroid 5-beta-reductase, or 5-beta-reductase, efficiently catalyzes the reduction of progesterone, androstenedione, 17-alpha-hydroxyprogesterone and testosterone to 5-beta-reduced metabolites.