T-cell surface glycoprotein CD4 (CD4) is an accessory protein for MHC class-II antigen/T-cell receptor interaction. It regulates T-cell activation, partially by recruiting the tyrosine kinase Lck, and induces the aggregation of lipid rafts. CD4 binds to HIV-1 gp120 and to P4HB/PDI, and upon HIV-1 binding to the cell membrane, is part of the P4HB/PDI-CD4-CXCR4-gp120 complex (1).