CXCR4 and infection: Cell fusions of syncytia formation are more possible for the CD4+ T-cells upon infection by the CXCR4-tropic HIV strains but not for cells that do not support productive infection of the replicating virus particles, such as the stem-progenitor cells (Koka et al., 1998; Koka et al., 1999; Zhang et al., 2010).