A strain of HIV that can enter and infect a host CD4 cell by attaching to either the CCR5 or CXCR4 coreceptor on the CD4 T lymphocyte (CD4 cell). To enter a CD4 cell, HIV must first attach to a CD4 receptor, then attach to either the CCR5 or CXCR4 coreceptor, and finally fuse its membrane with the CD4 cell membrane. HIV is usually R5-tropic (uses CCR5) during the early stages of infection, but the virus may later switch to using either only CXCR4 (X4-tropic) or both CCR5 and CXCR4 (dual-tropic).
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