CXCR4 and infection: In the original studies, Nef, a 27–34 kD myristoylated protein expressed early in the infection cycle in host cells [5], was shown to compete with stromal cell-derived factor 1-alpha (SDF-1α) a natural ligand of CXC chemokine receptor 4 (CXCR4), and induce apoptosis during the infection [6].