HLACs were pretreated with IL-21 and infected with replication competent CCR5-tropic (R5-HIV–green fluorescent protein (GFP)) or CXCR4-tropic (X4-HIV–GFP) HIV-1NL4-3-encoding green fluorescent protein (GFP) to allow for direct quantification of infection by flow cytometry (Supplementary Fig. 1). This evidence concerns the gene CXCR4 and infection.