A DNA sequence encoding the extracellular domain (Met1-Asp126) of human CD3E (NP_000724.1) was fused with the C-terminal polyhistidine-tagged Fc region of human IgG1 at the C-terminus, constructed the plasmid 1; A DNA sequence encoding the extracellular domain (Met1-Ser116) of human CD3G (NP_000064.1) was fused the Fc region of mouse IgG1 at the C-terminus, constructed the plasmid 2. The two plasmids were co-expressed and the human CD3E/CD3G heterodimer was purified.