CD4 and HIV-1 infection: In a humanized mouse model, CRISPR/Cas9-mediated reduction significantly decreased the surface expression of the CCR5 and CXCR4 co-receptors in primary CD4 + T cells, thereby establishing protection of the cells from HIV-1 infection by R5-tropic, X4-tropic, and dual R5/X4-tropic strains.