Replication of G431-mutated viruses. SupT1 cells were transfected with the T20-dependent (GIA-SKY, top panel) and the wild-type (GIV-SNY, lower panel) with variation at position 431 as indicated. Replication was measured both in the presence (100 ng/ml) and absence of T20. Virus replication curves were made over a 12-day period. Syncytia formation at day 12 was recorded (-, no syncytia; ++++, all cells involved in syncytia). Position 431 variation in the GIA-SKY T20-dependent virus (top) and the wild-type GIV-SNY virus (bottom): wild-type 431G (closed circles), G431R (open circles), G431E (dashes) and G431A (open triangles, in wild-type only).