Bifunctional enzyme involved in steroid-hormone metabolism and cholesterol biosynthesis (PubMed:9658408, PubMed:12732193, PubMed:12829805, PubMed:20659585). Catalyzes the NADP(H)-dependent reduction of estrogens and androgens and regulates the biological potency of these steroids. Converts estrone (E1)to a more potent estrogen, 17beta-estradiol (E2) (PubMed:9658408, PubMed:12732193). Converts moderately dihydrotestosterone (DHT) to their inactive forms 5a-androstane-3beta,17b-diol and 5alpha-androstane-3alpha,17beta-diol (PubMed:12732193). Does not metabolize progesterone (PubMed:12732193). Additionally, participates in the post-squalene cholesterol biosynthesis, as a 3-ketosteroid reductase (PubMed:12829805, PubMed:20659585). 17beta-estradiol + NADP(+) = estrone + H(+) + NADPH a 3beta-hydroxysteroid + NADP(+) = a 3-oxosteroid + H(+) + NADPH 3-dehydro-4alpha-methylzymosterol + H(+) + NADPH = 4alpha-methylzymosterol + NADP(+) H(+) + NADPH + zymosterone = NADP(+) + zymosterol 4alpha-methyl-5alpha-cholest-8-en-3-one + H(+) + NADPH = 4alpha-methyl-5alpha-cholest-8-en-3beta-ol + NADP(+) 4alpha-methyl-5alpha-cholest-7-en-3beta-ol + NADP(+) = 4alpha-methyl-5alpha-cholest-7-en-3-one + H(+) + NADPH 5alpha-cholest-8-en-3-one + H(+) + NADPH = 5alpha-cholest-8-en-3beta-ol + NADP(+) 5alpha-androstane-3alpha,17beta-diol + NADP(+) = 17beta-hydroxy-5alpha-androstan-3-one + H(+) + NADPH 5alpha-androstane-3beta,17beta-diol + NADP(+) = 17beta-hydroxy-5alpha-androstan-3-one + H(+) + NADPH Steroid biosynthesis; estrogen biosynthesis. Steroid biosynthesis; zymosterol biosynthesis; zymosterol from lanosterol: step 5/6. Binds to the short form of prolactin receptor. Colocalizes with HMGCR in a wide range of tissues during embryonic development. Most abundant in ovaries of pregnant animals. Present also in nonpregnant animals in ovaries, mammary gland liver, kidney and testis. Phosphorylated. Death at embryonic day 10.5. Embryonic lethality is caused by severe brain malformation and heart defect. Belongs to the short-chain dehydrogenases/reductases (SDR) family. ERG27 subfamily.