NS4 does not have a canonical nuclear localization signal (NLS) but possesses a stretch of basic amino acid residues, at the amino terminus portion of the protein, that could drive nuclear localization [41] (Figure 2D ). We constructed an NS4 expression plasmid (pNS4-GFP) and a series of deletion mutants (pNS4 7-77 -eGFP, pNS4 13-77 -eGFP and pNS4 19-77 -eGFP) lacking the 6, 12 and 18 amino terminal residues, respectively. pNS4-GFP and pNS4 7-77 -eGFP transfected cells showed a strong nuclear localization of NS4. On the other hand, NS4 showed a predominantly cytoplasmic localization in cells transfected with either pNS4 13-77 -eGFP or pNS4 19-77 -eGFP. These data suggest that the amino terminal basic domain of NS4 may play an important role in the nuclear localization of this protein.