Viral genotyping is used to determine the kind, or genotype, of the virus present. There are 6 major types of HCV; the most common (genotype 1) is less likely to respond to treatment than genotypes 2 or 3 and usually requires longer therapy (48 weeks, versus 24 weeks for genotype 2 or 3). Genotyping is often ordered before treatment is started to give an idea of the likelihood of success and how long treatment may be needed.