Steroid 5 Alpha Reductase 1 (SRD5a1) a member of the steroid 5α-reductase family (SRD5A1, SRD5A2 and SRD5A3) converting testosterone into 5-alpha-dihydrotestosterone and progesterone or corticosterone into their corresponding 5-alpha-3-oxosteroids. It plays a central role in sexual differentiation and androgen physiology. SRD5A1 was mainly expressed in the skin, scalp, liver, and brain tissues. Prostatic Adenoma and Prostatic Hyperplasia are associated with SRD5A1. Besides, 17-Beta-Hydroxysteroid Dehydrogenase Type 3 (HSD17b3) has been identified as an interactor of SRD5a1, thus a functional binding ELISA assay was conducted to detect the interaction of recombinant mouse SRD5a1 and recombinant human HSD17b3. Briefly, biotin-linked SRD5a1 were diluted serially in PBS, with 0.01% BSA (pH 7.4). Duplicate samples of 100 μl were then transferred to HSD17b3-coated microtiter wells and incubated for 1h at 37℃. Wells were washed with PBST 3 times and incubation with Streptavidin-HRP for 30min, then wells were aspirated and washed 5 times. With the addition of substrate solution, wells were incubated 15-25 minutes at 37℃. Finally, add 50 µl stop solution to the wells and read at 450 nm immediately. The binding activity of recombinant mouse SRD5a1 and recombinant human HSD17b3 was shown in Figure 1, the EC50 for this effect is 0.03 ug/mL.