Hepatitis C virus (HCV) is the major causative agent of chronic liver disease in developed countries. HCV is classified into six genotypes, corresponding to the major branches of the phylogenetic tree constructed from alignments of the HCV complete genomes. Smaller branches of the tree correspond to subtypes. The genotypes are numbered 1 to 6, and the subtypes are designated by numbers and letters (e.g., 10a). Recent studies have shown that the effectiveness of therapeutic treatment correlates with the genotype of the virus.