NP-2/CD4/FPRL1 cells were found to be susceptible to the GUN-1WT, GUN-4V, and GUN-7WT cell-line-adapted HIV-1 strains: approximately 0.5, 5 and 30% of the cells became HIV-1 antigen-positive on day 6 after infection, respectively (Fig. 3A). Here, CD4 is linked to infection.