CD4 and infection: NP-2/CD4/CXCR4 cells were highly susceptible to all HIV-1 strains, except the Ba-L and SF162 strains, when tested on day 6 after infection (Fig. 3B), while NP-2/CD4/CCR5 cells were highly susceptible to five HIV-1 strains, Ba-L, GUN-1WT, GUN-4WT, GUN-7WT, and SF162, but not to the IIIB strain (Fig. 3C).