CXCR4 and infection: Consistently, CXCR4 KO cells were completely resistant to X4 HIV-1 green fluorescent protein (GFP) infection; <0.01% versus 7.7% of NTC cells expressed GFP reflecting viral gene expression from reverse-transcribed, integrated HIV-1 genomes 3 d after viral challenge (Fig. 3f) under conditions of highly efficient X4 HIV-1 fusion (Extended Data Fig. 6b).