The nucleotide composition of the 5′UTR of the yellow fever virus (YFV) has been reported to be the most constant sequence in the viral genome, but a comprehensive analysis of this constancy has not been presented. The current report is an analysis of the 5′UTRs from 48 sequences deposited in GenBank representing the seven described genotypes, five in Africa and two in the Americas. The YFV 5′UTRs consist of 118-120 nucleotides, 92% (110/119) of which are constant in all sequences. The

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... ences. The constancy is impressive and suggests that many participate in significant viral functions. Remarkably, analysis of the non-constant nucleotides revealed that in some instances the non-constant nucleotide changes persisted in one or a restricted number of related genotypes and were from sequences isolated over a considerable span of years. This constant feature of non-constant nucleotides is consistent with the concept that the changes were in response to different environmental features such as changes in mosquito hosts or animal reservoirs, particularly as a consequence of spread of YFV from Africa to the New World. Constancy of 5′UTR in general may be helpful in distinction of viral species. Lastly, the presence of sequences of constant nucleotides greater than 19 nucleotides suggests regions of the 5′UTR that may be exploited for use as non-codon RNA as treatment and diagnostic agents in a variety of viral diseases.