DNA vaccine pCAGGS-NP was constructed and assessed according to Ma and Jiang [24, 25] . The synthetic gene for NP of the isolate A/Goose/Gongdong/1/96 (H5N1) with codons optimized for chicken usage was synthesized by PCR assembly of long single-strand DNA templates (100 bases in length) (the oligonucleotide sequences are available upon request). The synthetic optiNP was cloned into the plasmid vector pCAGGS under the control of the chicken b-actin promoter. The plasmid was named pCAGG-NP. The expression of the NP protein from the plasmid was confirmed by indirect immunofluorescence assay and Western blotting of plasmid transfected CEF cells.