CXCR4 and infection: Because NL4–3 replicates very efficiently in PBMC culture, we added AMD3100 (20 nM), a CXCR4 inhibitor specific to this virus (i.e., one that would not affect the R5 virus CC101.19 cl.7), to reduce its replication rate to a level comparable to that of its competitor in the dual-infection cultures.