In this paper, we concluded that removal of the SV5 F protein cytoplasmic tail (F tail or FΔ19) caused a block in fusion promotion at the hemifusion stage. Further examination, using more modern and sophisticated procedures involving double labeling and fluorescent dyes of different sizes, has shown that, although the F tail mutant is severely destabilized in the formation of syncytia relative to wild-type F protein, the F tail mutant is capable of promoting the transfer of a small aqueous dye, suggesting that a small fusion pore opens. Thus, the mutant is not blocked at the hemifusion stage. (Dutch, R.E., and R.A. Lamb. 2001. J. Virol. 75:5363–5369.)