CCR5 and HIV-1 infection: Fig. 3D shows that although all these cells exhibit similar RLA when infected with HIV-1 X4-tropic Bru3, when infected with the five R5-tropic transmitted founder (T/F) HIV-1 isolates, WITO, CH040, THRO, REJO and CH106 [38], significantly lower RLA was found in CCR5-/CR1, CCR5-/CR2 and CCR5-/CR3 cells than in mock-transduced TZM.bl cells. Taken together, we conclude that cells with disrupted CCR5 genes are highly resistant to R5-tropic HIV-1 infection.