CD81 and infection: To determine whether the surface level of CD81 and the sensitivity to HCV infection can be differentially controlled by high and low levels of HCV replication in cell cultures without the use of drug selection, we compared the downregulation of CD81 surface expression and the resistance to HCVpp infection between JFH1-tranfected cells and cells transfected with the J6/JFH1, a chimeric viral genome consisting of the structural genes (C, E1, and E2), p7, and NS2 from another genotype 2a strain (strain HC-J6) and the NS2 to 3′-UTR fragment of JFH1.