In fact, CD34+ HPCs have been shown to secrete the CCR5 ligands MIP-1α, MIP-1β, and RANTES [69],[70], and the CXCR4 ligand SDF-1 [60], which may compete with infectious HIV-1, either CCR5- or CXCR4-utilizing virus, and consequently block infection. Here, CXCL12 is linked to infection.