Bone Morphogenetic Protein 9 (BMP9), a member of the Transforming Growth Factor Beta(TGF-b) superfamily, is predominantly expressed in the liver and endothelial cells. It exerts pivotal roles in regulating angiogenesis, glucose metabolism, and bone formation. As a multifunctional cytokine, BMP9 transmits signals via type I and type II serine/threonine kinase receptors on the cell surface, thereby modulating a variety of physiological and pathological processes. Notably, it binds to Activin A Receptor Type II A (ACVR2A) with high affinity, which initiates downstream SMAD-dependent signaling pathways.Thus a functional binding ELISA assay was conducted to detect the interaction of recombinant mouse BMP9 and recombinant human ACVR2A . Briefly, biotin-linked BMP9 were diluted serially in PBS, with 0.01% BSA (pH 7.4). Duplicate samples of 100μl were then transferred to ACVR2A-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 50μl stop solution to the wells and read at 450nm immediately. The binding activity of recombinant mouse BMP9 and recombinant human ACVR2A was shown in Figure 1, the EC50?for this effect is 0.17 μg/mL.