Clarification required for the definition of hepatitis B virus subgenotypes C1 and C2.

To the Editor Hepatitis B virus (HBV) has been classified into 8 major genotypes A to H, with an intergenotypic diversity of at least 8% in the full genome sequence. Genotypes A to D are the most common worldwide, with genotype A predominant in Western Europe, North America and Africa, genotypes B and C most frequent in Asia, and genotype D present in Mediterranean countries [1] . There is increasing evidence that HBV strains, even within the same genotype, differ in virological and clinical manifestation. Within the HBV genotypes, variability of between 4 and 7.5% has led to further classification into subgenotypes [1] , although HBV subgenotypes are yet to be ratified by the International Committee for the Taxonomy of Viruses (ICTV) [2] . The HBV genotype C is prevalent in Asia, where it causes more serious liver disease than genotype B [3] . In the 2004 report of Norder et al. [4] , phylogenetic analysis of 66 complete genotype C genomes identified 4 groups of genotype C (C1–C4), with clear geographical clustering. The C1 subgenotype consisted of HBV strains from East Asia, including Japan, Korea and China, whilst the C2 subgenotype dominated in China, Thailand, Laos, Vietnam and Bangladesh. The C3 subgenotype was confined to New Caledonia and Polynesia and subgenotype C4 was confined to Australian aborigines in Northeast Australia. Subsequently, a C5 subgenotype has also been identified in the Philippines [5] . Published online: September 17, 2009