To investigate minimal unit of the epitope with the maximal binding activity to mAb A9E8, a series of GST-fusion proteins were expressed with extended amino acid residues at both N and C termini of the motif SVQYHPL (Table 1) . These GST-fusion proteins were subjected to SDS-PAGE and testing for reactivity with mAb A9E8 in Western blot. Fusion proteins GST-R1 (SVQYHPLA), GST-R2 (SVQYHPLAL) and GST-R3 (SVQYH-PLALP) reacted strongly with mAb A9E8 in Western blot ( Figure 4) . The GST-R2 and GST-R3 showed similar binding activity to the GST-R1, indicating that alanine alone significantly increased binding activity of the core epitope to mAb A9E8. In contrast, GST-fusion proteins with extended amino acid residues at the N terminus of the motif SVQYHPL showed no increased binding activity compared with GST-H7wt in Western blot (data not shown). Taken together, these results showed that SVQYH-PLA was the minimal unit of the epitope with the maximal binding activity to mAb A9E8.