CD4 and HIV-1 infection: Having shown that the levels of CD4, the principal cellular receptor of HIV-1, could be maintained upon differentiation of the THP-1-CD4R cell line, we analyzed the susceptibility of these cells to HIV-1 infection and importantly examined whether HIV-1 functions that are phenotypically assessable only in primary macrophages could be studied in the THP-1-CD4R cell line.