Additional evidence supporting the conventional pathway for cis infection of LC by either R5 or X4 strains is that this can be blocked by pretreatment of the cells with natural and synthetic ligands for CCR5 (RANTES; regulated on activation, normally T cell-expressed and secreted) or CXCR4 (SDF-1; stromal cell-derived factor 1) [49, 50], as well as blockers of gp41 fusion [51]. This evidence concerns the gene CXCL12 and infection.