Alternately, CXCL12 impairs HIV-1 infection by CXCR4 tropic virus by competitively binding CXCR4 and blocking interactions with gp120 (Bleul et al., 1996; Oberlin et al., 1996; Amara et al., 1997; Kaul and Lipton, 1999). The gene discussed is CXCR4; the disease is HIV-1 infection.