HIV strains that infect host cells using the CCR5 coreceptor are known as M-tropic, non-syncytium-inducing, CCR5-tropic (R5) viruses, while infections via the CXCR4 coreceptor occur with T-tropic, syncytium-inducing, CXCR4-tropic (X4) HIV strains. Here, CXCR4 is linked to infection.