Moreover, Conrad Bleul’s work demonstrated that only CXCL12/SDF-1 including an intact amino-terminal domain was active as suppressor factor for infection by T-tropic viruses, indicating that the amino-terminal domain in CXCL12/SDF-1 was required for both activation of CXCR4 and inhibition of T-tropic HIV-1 species. This evidence concerns the gene CXCR4 and infection.