CD4 and infection: The decreased levels of HIV-1YU2 and HIV-1HXB2 infection of TZM-BL cells expressing siRNA targeted to Tiam-1, Abl, IRSp53, Wave2, and Arp3 were comparable to levels of Env-mediated cell fusion with U87.CD4.CCR5 cells expressing these siRNAs, whereas siRNA to Trio had no effect (Figure 3B, top two panels).