Locations of HIV-1 replication and latency and routes of transmission between haematopoietic cell populations. All cell types shown are susceptible to HIV-1 entry and integration of the proviral DNA. Some anatomical locations are shown; those outside of marked areas are in the bloodstream, lymphatic system and/or tissues. Black arrows represent differentiation and/or maturation and may represent more than one step and could involve multiple intermediate cell types. Purple arrows represent routes of trans infection, and relative rates are shown as high or low. "Rep" indicates productive HIV-1 replication with relative rates shown as high or low. HIV-1 cis infection routes are not shown, as any susceptible cell may be infected by productive replication from another cell. Those cells in which HIV-1 latency is thought to occur should be considered as putative viral reservoirs and therapeutic targets.