Small calcium binding protein that S100 Calcium Binding Protein (S100), also known as S100 Alpha (S100A1), is a member of the S100 family of calcium-binding proteins. As with most S100 proteins, S100A1 proteins are localized in the cytoplasm and/or nucleus of a wide range of cells, and it possess a variety of intracellular and extracellular functions. They interact with multiple receptors and signal transducers to regulate pathways that govern inflammation, cell differentiation, proliferation, energy metabolism, apoptosis, calcium homeostasis, cell cytoskeleton and microbial resistance. S100 Calcium Binding Protein A4 (S100A4) is one of targets of S100A1. Thus a functional binding ELISA assay was conducted to detect the interaction of recombinant rat S100A1 and recombinant mouse S100A4. Briefly, S100A1 was diluted serially in PBS with 0.01% BSA (pH 7.4). Duplicate samples of 100 μl were then transferred to S100A4-coated microtiter wells and incubated for 1h at 37℃. Wells were washed with PBST and incubated for 1h with anti-S100A1 pAb, then aspirated and washed 3 times. After incubation with HRP labelled secondary antibody for 1h at 37℃, 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 450/630 nm immediately. The binding activity of recombinant rat S100A1 and recombinant mouse S100A4 was shown in Figure 1, the EC50?for this effect is 3.92 ug/mL.