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The measles virus is classified in the paramyxovirus family, genus Morbillivirus. It con-tains linear, negative-sense, single-stranded RNA, which encodes at least six virion struc-tural proteins. Of these, three are in the envelope, comprising a matrix (M) protein that plays a key role in viral assembly and two types of glycoprotein projections (peplomers). One of the projections is a hemagglutinin (H), which mediates adsorption to cell surfaces; the other (F) mediates cell fusion, hemolysis, and viral entry into the cell. No neur-aminidase activity is present. The receptor for measles virus is CD46 (membrane cofactor protein), a regulator of complement activation. Only a single serotype restricted to human infection is recognized; however, subtle antigenic and genetic variations among wild type measles strains do occur. These variations can be determined by sequencing analyses, en-abling more precise epidemiologic tracking of outbreaks and their origins. Such ongoing molecular surveillance is also extremely important in determining whether significant anti-genic drifts evolve over time.
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