CD4 and HIV-1 infection: To evaluate virus–cell contact regions potentially involved in infection events more unambiguously, we pre-incubated uninfected CD4+ T cells susceptible to SIV and HIV-1 infection with high concentrations of infectious virus at 4 °C, then warmed the cells to 37 °C and carried out rapid fixation after waiting for periods ranging from 15 min to 3 h.