CXCR4 and infection: There was a severe genetic bottleneck after intravenous injection of NFNSX-BC, restricting the number of viral RNA barcodes that established early infection in vivo with a subsequent loss in number and diversity and increase in dominance of rebounding viral RNA barcodes after ART was discontinued (Fig. 1d–g, Supplementary Data S1), which was similar to previous in vivo studies using a barcoded CXCR4 (X4)-tropic and vpr-deficient HIV-133,34.