CD4 and infection: HIV-1 p24 levels in the supernatants of infected cells were measured at 1, 3, 5 and 7 days post-infection and the results indicated that lenti-X4R5-Cas9 modified primary CD4+ T cells were protected from infection by either X4- or R5-tropic HIV-1 strains, compared to unmodified control (Fig. 5c).