CXCR4 and HIV-1 infection: One noteworthy feature of the midbrain and the substantia nigra specifically that may help explain the susceptibility of this brain region to HIV-1-related damage is that this area contains a high concentration of microglial cells [92], which are the only brain cell types that can support productive HIV-1 infection, express CXCR4 and CCR5 and are a rich source of inflammatory mediators (O2- and cytokines/chemokines) [8-11].