Epidemiological^ unrelated isolates of the same HSV serotype are not identical (intratypical polymorphism). Differences result from base substitutions that add or eliminate a restriction endonuclease cleavage site or substitute an amino acid, occasional deletions, or due to variability in the number of repeated sequences at various genome sites. Intratypic polymorphism was also noted with monoclonal antibodies and isolates may differ with respect to neurovirulence. The restriction endonuclease cleavage patterns of a given strain are relatively stable even after many years of serial in vitro passage.
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