Hepatitis C virus (HCV) is a member of the Flaviviridae family. Its genome is a positive single-stranded RNA molecule which comprises three distinct regions: a 5' non coding region, a long open reading frame encoding both the structural and non structural viral proteins, and a 3' non coding region. HCV circulates in infected individuals as complex mixtures of genetically distinct but closely related variants referred to as "quasispecies". The quasispecies nature of HCV genomes appears to play a major role in viral persistence, cell tropism of viral variants, pathogenicity and resistance to antiviral therapy.