Vitamin D Receptor (VDR) is the nuclear hormone receptor for vitamin D3. The receptor belongs to the family of trans-acting transcriptional regulatory factors and shows sequence similarity to the steroid and thyroid hormone receptors. It mediates the action of vitamin D3 by controlling the expression of hormone sensitive genes. Besides, Retinoid X Receptor Alpha (RXRa) has been identified as an interactor of VDR, thus a binding ELISA assay was conducted to detect the interaction of recombinant human VDR and recombinant human RXRa. Briefly, VDR were diluted serially in PBS, with 0.01% BSA (pH 7.4). Duplicate samples of 100 ul were then transferred to RXRa-coated microtiter wells and incubated for 1h at 37℃. Wells were aspirated and incubated for 1h with anti-VDR pAb, then aspirated and washed 3 times. After incubation with HRP labelled secondary antibody, 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 450nm immediately. The binding activity of of VDR and RXRa was shown in Figure 1, and this effect was in a dose dependent manner. The EC50 for this effect is 0.469 ug/mL.