CD4 and infection: We next sought to establish, for the evolving R5 viruses in BR24, an association between sCD4 sensitivity, soluble gp120 (sgp120) binding to CD4-Ig, infection of primary macrophages and HeLa RC49 cells, the latter having been used as an indicator of macrophage-tropism and ability to utilize low levels of CD4 for infection [43], [76].