Macrophage (M-tropic) strains of HIV-1, or non-syncytia-inducing strains (NSI; now called R5 viruses[25] ) use the β-chemokine receptor CCR5for entry and are, thus, able to replicate in macrophages and CD4+ T cells. This CCR5 coreceptor is used by almost all primary HIV-1 isolates regardless of viral genetic subtype