2001b and personal communication; Steven Feinstone, David Thomas, Jian-Qiu Han, Binhua Ling, Jinhua Xiang and Jack Stapleton). Thus, it is not clear if organisms other than humans and chimpanzees are susceptible to HCV and GBV- C/HGV infection. In contrast, the natural hosts of GBV-A and GBV-A-like variants include at least six species of New World monkeys including Saguinus species (Saguinus labiatus, Saguinus mystax, Saguinus nigricollis and Saguinus oedipus), Callithrix species (Callithrix jacchus) and Aotus species (Aotus trivirgatus) (Bukh & Apgar, 1997; Leary et al., 1996a; Muerhoff et al., 1995; Simons et al., 1995b). The 59 non- translated region (NTR) and NS3 helicase sequences of GBV-A isolates from different host species were found to segregate into distinct groups, suggesting co-speciation of these viruses with their natural hosts (Bukh & Apgar, 1997; Leary et al., 1996a; Muerhoff et al., 1995; Simons et al., 1995b). In contrast, no natural host for GBV-B has been identified. Experimental GBV-B infection of tamarins and aotus monkeys have been documented, but experimental inoculation into chimpanzees did not provide evidence of viral replication (Bukh et al., 2001a). Identification of the natural host of GBV-B may be complicated by the short duration of viraemia and the lack of a reliable serological method to detect prior infection (Pilot-Matias et al., 1996b; Schaluder et al., 1995). Alternatively, tamarins may not be the natural host of GBV-B, and the virus may indeed be capable of establishing persistent infections in an altern- ative, natural host. GBV-D has only been reported in one bat species (Epstein et al., 2010). A summary of the host range, tropism and pathogenesis is presented in Table 1. Genome organization Like other members of the family Flaviviridae, HCV, GBV- A, GBV-B, GBV-C/HGV and GBV-D have a positive-sense, ssRNA genome that contains a single long ORF encoding a multifunctional polyprotein (Fig. 1) (Birkenmeyer et al., 1998; Choo et al., 1991; Kim & Fry, 1997; Leary et al., The GB viruses 1996b; Muerhoff et al., 1995). The 59 NTR contains an internal ribosomal entry site (IRES) element that directs translation of the polyprotein directly from viral genomic RNA (Simons et al., 1996; Yoo et al., 1992), and translation and release of the viral RNA-dependent RNA polymerase (RdRp) initiates RNA replication (reviewed by Major & Feinstone, 1997; Mohr & Stapleton, 2009; Moradpour et al., 2007; Robertson, 2001). The viral structural proteins are processed from the amino-terminal portion of the polyprotein by cellular signal peptidases or signal peptide peptidase, while the NS proteins are processed by two viral encoded proteases. An NS2–NS3 autoprotease catalyses the cleavage of NS2–NS3 for HCV, and based on the conservation of two essential amino acids for HCV NS2 function (His-952 and Cys-993), NS2-mediated cleavage of NS2–NS3 is thought to occur in GBV-A, GBV-B, GBV-C/ HGV and GBV-D viruses as well. The remaining NS proteins are cleaved by the NS3 protease with its co-factor NS4A in HCV, GBV-B, GBV-C/HGV and presumably GBV-A and GBV-D (reviewed by Epstein et al., 2010; Major & Feinstone, 1997; Mohr & Stapleton, 2009; Moradpour et al., 2007; Robertson, 2001). Like NS2, NS3 amino acids essential for the processing of HCV NS proteins (His-1083, Asp-1107 and Ser-1165) are present in all four GB viruses and HCV. Their NS3 proteases are classified as chymotrypsin-like serine proteases (reviewed by Major & Feinstone, 1997; Mohr & Stapleton, 2009; Moradpour et al., 2007; Robertson, 2001). Although HCV and the four GB viruses have somewhat similar genome organization and predicted protein struc- ture, there are distinct differences. All of the GB viruses studied have an IRES element in the 59 NTR, although their structures differ, with GBV-B having a type 3 IRES, like HCV, while the IRES element in GBV-A and GBV-C/HGV conform better with type 4 IRES elements (Kieft, 2008). However, others state that the GBV-A and GBV-C/HGV IRES do not conform to any recognized IRES class (Bakhshesh et al., 2008). IRES activity has not been examined in GBV-D. Direct comparative data do not exist Table 1. Comparison of tropism, host range and pathogenesis of GB viruses and Hepaciviruses IDA, Insufficient data available; NWP, New World primates. Virus Pegivirus GBV-C GBV-Ccpz GBV-A GBV-D Hepacivirus HCV GBV-B Proposal* Tropism Host range Pathogenesis HPgV SPgVcpz SPgV BPgV HCV GBV LymphocytesD IDA LymphocytesD IDA Hepatoctyes Hepatocytes Humans, chimpanzee Chimpanzee NWP Bats Humans, chimpanzee NWP None identified None identified None identified IDA Hepatitis Hepatitis *Genus (italics) and virus nomenclature proposed in this review. DData support lymphotropism, but other sites of replication are possible. http://vir.sgmjournals.org 237