A DNA sequence encoding the human S100A8 (NP_002955.2) (Met1-Glu93) was fused with a polyhistidine tag at the C-terminus, constructed the plasmid 1; A DNA sequence encoding the human S100A9 (NP_002956.1) (Met1-Pro114) was fused with a flag tag at the C-terminus, constructed the plasmid 2. The two plasmids were co-expressed and the S100A8 & S100A9 heterodimer was purified.