At the same time, simultaneous knockout of both HIV co-receptors Chemokine C-C-Motif Receptor 5 (CCR5) and CXCR4 leads to a decrease in the expression of CCR5 and CXCR4, which makes the modified cells resistant to infection with R5 and X4 tropic viruses, even when using double tropic viruses [230]. Here, CCR5 is linked to infection.