CD4 and HIV-1 infection: We have recently developed a CRISPR/Cas9-based fusion inhibitory peptide knock-in (KI) into the CXCR4 locus and showed that the MT-C34 peptide protects the CEM/CCR5 T cell line or primary CD4+ T lymphocytes from HIV-1 infection when expressed on their surface in a context of a GPI-anchored protein CD52 [22].