NP-2/CD4/CXCR4 cells showed a high susceptibility to AG204, AG206, AG208, HCM303, HC305, HCM308, and HCM309 isolate (Fig. 4C), while less than 1% of HIV-1 antigen-positive cells were detected after infection with HCM342, mIDU101, or mSTD104 isolates. Here, CD4 is linked to infection.