CCR5 and infection: Cells challenged with the X4-tropic HIV-1NL4-3 showed high levels of p24 HIV-1 at day 11 post-infection in the single CRISPR/Cas9 knockout CCR5 (1,394,774 pg/ml ± 27,923; mean ± SD) and the non-anti-viral control cells (1,081,368 pg/ml ± 15,285; mean ± SD), suggesting that only the effects of CRISPR/Cas9 knockout CCR5 alone do not prevent the X4-tropic HIV-1NL4-3 infection.