Gp120 is critical for virus infection, as the protein is necessary for binding to specific cell surface receptor CD4, mainly expressed on CD4+ T cells, and the coreceptor [e.g., chemokine receptor C-C Motif Chemokine Receptor 5 (CCR5) or C-X-C chemokine receptor type 4 (CXCR4)] (138). Here, CD4 is linked to viral infectious disease.