West Nile virus (WNV), a member of the genus Flavivirus, is a mosquito-borne RNA pathogen closely related to numerous other flaviviruses of the Japanese encephalitis (JE) group, and to a lesser degree to Dengue virus (DENV), Yellow fever virus (YFV), and several other flaviviruses of about 70 different species (5, 18). The genome is a positive-sense, single-stranded RNA of approximately 10,293 nucleotides encoding a polyprotein of approximately 3,430 amino acids (aa) that is cleaved to produce three structural proteins, capsid (C), precursor membrane (prM), and envelope (E), and seven nonstructural (NS) proteins, NS1, 2a, 2b,3, 4a, 4b, and 5 (24,37). Originally isolated in Africa in 1937 (62), WNV has become an increasingly important human pathogen widely endemic in Africa, Asia, Europe, and North America and a significant agent of viral encephalitis (28,38,45). WNV and several of the major human pathogen flaviviruses are known to cocirculate (28, 71); for example, WNV and St. Louis encephalitis virus (LEV) in North America; WNV, DENV, and Japanese encephalitis virus (JEV) around the Indian subcontinent and portions of Southeast Asia (SEA); and WNV, DENV, JEV, and Murray Valley encephalitis virus (MVE) in neighboring pockets of SEA and Australasia.The widespread colocalization of WNV with other flaviviruses calls for a greater understanding of the phylogenetic relatedness and possible pathophysiologic associations of flaviviruses. We previously reported a large-scale analysis of the conservation and variability of overlapping nonamers, the typical length of human leukocyte antigen (HLA) class I or class II binding cores of T-cell epitopes (47), of the 2,746 WNV protein sequences collected from the NCBI Entrez Protein Database (17). Notably, of the 88 completely conserved sequence fragments identified, representing 34% of the WNV proteome, 67 were also present in many other flaviviruses with identities of nine or more amino acids. These sequence homologies called attention to a broad inter-Flavivirus risk of altered peptide ligands (APL) (58), T-cell epitopes with one or more amino acid differences, that possibly would result in modified immune responses in the event of exposure to multiple Flavivirus pathogens.This study focused on analyses of the diversity and presence in