HCV infectivity determination by density gradient analysis and infection kinetics assay. (a). Density gradient analysis of produced HCV particles. Concentrated culture medium collected from HDAdJFH1-infected Huh-7 cells was fractionated, HCV RNA levels and HCV infectivity titers in each fraction was determined. (b). Infection of naïve Huh-7 cell by the supernatant from the HDAd-JFH1-infected Huh-7 cells and infection kinetics assay analysis.