Figure. Genetic relationship between measles viruses isolated in the United States in 1997–2001 and the reference strains established by the World Health Organization (WHO) (20). Phylogenetic tree was based on the nucleotide sequences coding for the COOH-terminus of the nucleoprotein. Strain abbreviations are given in Table 1. Reference strains as established by WHO are shown in bold and designated by their genotype name. The length of the horizontal scale bar represents one nucleotide change.
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