CD4 and infection: We worked with a prototypic HIV-1 that uses CD4 and CCR5 cell-surface coreceptor to enter cells and is typical at the early stages of infection (isolate BaL) and a prototypic HIV-1 that uses CD4 and CXCR4 cell-surface coreceptor that often evolves at the late stages of infection (isolate LAI.04).