CD4 and infection: Env variants interacting with CD4/CXCR4 (rather than those having a preference for CD4/CCR5) are mostly encoded by syncytium-inducing HIV-1 strains, and a strong correlation between CD4+ T-cell decline and infection by syncytium-inducing HIV-1 variants has been established.16 Many distinct viruses express on their plasma membrane surface glycoproteins that interact with receptors on target cells and facilitate virus–cell fusion.