Flavivirus virions are $50-nm particles with a nucleocapsid composed of capsid (C) protein surrounding a positive-sense single-stranded RNA genome of $11 kb. The capsid is enclosed in a lipid membrane within which the viral membrane (M) and envelope (E) proteins are embedded. The genome encodes a single polyprotein of approximately 3400 amino acids from which the three structural (C, M and E) and seven non-structural (NS1, NS2A, NS2B, NS3, NS4A, NS4B and NS5) proteins are processed by cellular and viral proteases (8) .