CD4 and infection: When NP-2/CD4/FPRL1 cells were exposed to these isolates, the cells were found to be clearly susceptible to AG204, AG206, HCM308, HCM342, and mSTD104 isolates: 20, 30, 15, 40, and 60% cells, respectively, became HIV-1 antigen-positive by IFA on day 6 after infection and syncytia were formed (Figs. 4A and 5).