CD4 and infection: Upon infection of specific cells, HIV-1 utilizes the viral surface protein gp120 to bind with the host cell surface receptor CD4, and then, gp120 undergoes conformational alteration to expose gp41, which facilitates fusion between virions and cell membranes, allowing the release of the viral core particle into the cytoplasm for genome replication (Chen, 2019; Christensen et al., 2020).