Results showed that the TSLF-CD4-CCR5 cells exhibited p24 levels as high as those found in human HOS-CD4-CCR5 cells with R5-tropic HIV-1Bal infection, whereas the human CD4- and human CCR5-negative tree shrew cells did not allow infection of R5-tropic envelopes (HIV-1Bal) (Fig. 3C). Here, CD4 is linked to infection.