[Molecular virology of TT virus]

In 1997, a novel DNA virus was isolated from the serum of a patient with posttransfusion hepatitis of unknown etiology in Japan, and it was named TT virus (TTV) after the initials of the index patient. TTV is a nonenveloped, single-stranded and circular DNA virus, and its entire nucleotide sequence of 3.9 kb has been determined. For being a DNA virus, TTV has a wide range of sequence divergence, allowing the classification into at least 16 genotypes separated by an evolutionary distance of > 0.30. Nucleotide sequence of the noncoding region is conserved, whereas the coding region sequence is highly variable. TTV strains with extremely high sequence divergence prevail in infected individuals. An association of TTV genotypes, detectable by PCR with N22 primers or genotype 1-specific primers, with hepatitis of unknown etiology suggest that TTV of restricted genotypes would be clinically important.