Effect of hCD81 expression on cell fusion. CHO cell lines expressing either or both of the chimeric E1 and E2 proteins (CHOE1, CHOE2, and CHOE1E2, respectively) and the parent cell line (CHO) were cocultured with the NIH 3T3 cell line constitutively expressing hCD81 or the parent cell line. As a positive control, Hep G2 cells were used. Cell fusion activity was determined as described for Fig. 2. RLU, relative light units.