CXCR4 and infection: Consistent with the results of viral entry assays on Cf2-Luc cells, both C34-CCR5 and C34-CXCR4 were highly resistant to BaL and HxB2 infection (Fig 1B) indicating that the amino terminal HR2 peptide was likely able to access and inhibit HR1 domains of the pre-hairpin intermediate to block 6 helix-bundle formation and viral entry.