A DNA sequence encoding the p35 subunit of human IL-12, termed as IL12A (NP_000873.2) (Met1-Ser219) was fused with a polyhistidine tag at the C-terminus, constructed the plasmid 1 ; A DNA sequence encoding the p40 subunit of human IL-12, termed as IL12B (NP_002178.2) (Met1-Ser328) was fused with a polyhistidine tag at the C-terminus, constructed the plasmid 2. The two plasmids were co-expressed and the heterodimer was purified.