Fibroblast Growth Factor 9 (FGF9) is a member of the fibroblast growth factor (FGF) family. FGF family members possess broad mitogenic and cell survival activities and are involved in a variety of biological processes, including embryonic development, cell growth, morphogenesis, tissue repair, tumor growth and invasion. FGF9 was isolated as a secreted factor that exhibits a growth-stimulating effect on cultured glial cells. In nervous system, this protein is produced mainly by neurons and may be important for glial cell development. FGF9 secreted by HSC is a candidate activating ligand for hepatocyte FGFR4 functions, thus a functional binding ELISA assay was conducted to detect the interaction of recombinant human FGF9 and recombinant human FGFR4. Briefly, biotin-linked FGF9 were diluted serially in PBS, with 0.01% BSA (pH 7.4). Duplicate samples of 100 ul were then transferred to FGFR4-coated microtiter wells and incubated for 1h at 37℃. Wells were washed with PBST 3 times and incubation with Streptavidin-HRP for 30 min, 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 FGF9 and FGFR4 was shown in Figure 1, the EC50 for this effect is 0.23 ug/mL.