Effect of fHR1 on fMHV-mediated fusion processes. (a) Preincubation of fMHV with fHR1 blocked the infection of feline FCWF-4 cells. (b) In addition, the formation of syncytia was inhibited when fHR1 peptide was added to the culture medium 1 h after infection of FCWF-4 cells. (c) fHR1 also caused a significant reduction in the number of infected human A431 cancer cells when inoculated with a preincubated mixture of fMHV and scFv 23F-425 in the presence of 0.5 μM fHR1. (d) Also the cell–cell fusion was inhibited by fHR1 when peptide and scFv 23F-425 were added to the medium of A431 cells that had been infected with a preincubated mixture of fMHV and scFv 23F-425. All data shown represent the average and standard deviation of an experiment performed in triplicate.