Human herpesvirus (HHV)-6 strains segregate into two variants (HHV-6A and HHV-6B), closely related to each other but clearly and easily distinguishable. These two HHV-6 variants differ in their ability to grow in T-cell lines, have distinctive patterns of DNA restriction fragments, and show specific reactivities with some monoclonal antibodies. The degree of DNA homology between variants ranges from 97% in the most conserved region to 75% in the immediate early region 1. HHV-6B is the etiologic agent of exanthema subitum but HHV-6A has not yet been clearly associated with any human pathology. HHV-6 sequences are frequently detected by the polymerase chain reaction (PCR) in healthy and pathological tissues. HHV-6B is more prevalent in peripheral blood mononuclear cells and in lymphatic tissue. The prevalence of HHV-6A may be greater in some pathological conditions such as Kaposi's sarcoma, and in skin biopsies. Results so far available support the hypothesis that HHV-6 variants may have different epidemiologies.
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