By 1960 it was recognized that eastern equine encephalitis, western equine encephalitis, and certain other arboviruses were related to each other. These were categorized as “Group A arboviruses”, now known as the genus Alphavirus of the family Togaviridae. As yellow fever virus (YFV), Japanese encephalitis, Murray Valley encephalitis, St. Louis encephalitis, West Nile, Louping ill, Russian spring–summer encephalitis, and other viruses were also known to be interrelated antigenically, they were termed “Group B arboviruses,” now known as the genus Flavivirus of the family Flaviviridae. Current taxonomic placement of Flaviviruses comprises one of three genera within the virus family Flaviviridae. The other two are the genera Pestivirus and Hepacivirus. An insect virus known as Cell-Fusing Agent had also been placed informally and tentatively in the genus Flavivirus on the basis of its similar genome strategy and partial homology with these viruses. Genomic RNA of viruses of the three genera is similarly organized and is the only mRNA found in infected cells. It contains a single long open reading frame flanked by 50- and 30-terminal noncoding regions that form specific secondary stem–loop structures required for genome replication. In flaviviruses, translation initiation is cap–dependent, whereas internal ribosomal entry sites have been demonstrated for pestiviruses and hepaciviruses.