Sequence variability in the putative coding region of TT virus: evidence for two rather than several major types.

Recently a new human virus, TT virus (TTV) was identified in the serum of a patient with post-transfusion hepatitis of unknown aetiology. Comparative sequence analysis of a 222 nt fragment of ORF 1 of TTV was performed to assess the genomic variability of this virus. Phylogenetic analysis of the nucleotide sequences of 76 TTV isolates collected in 17 countries segregated them into two major groups: TTV 1 and TTV 2. The TTV 1 group comprised two distinct subgroups, which corresponded to previously described TTV subtypes 1a and 1b. The TTV 2 group was separated into four main branches, two of which included sequences previously provisionally attributed as TTV types 2 and 3. Bootstrap resampling, however, did not support the reliability of this grouping, suggesting that the isolates in the TTV 2 group should be considered as subtypes of a single type rather than different TTV types.