GP41 is a glycoprotein subunit that remains non-covalently-bound to gp120, and provides the second step by which HIV enters the cell. It is originally buried within the viral envelope, but, when gp120 binds to a CD4, gp120 changes its conformation, causing gp41 to become exposed, where it can assist in fusion with the host cell. Alternative Names: gp41 antigen, HIV 1 gp41, HIV-1 gp41 protein, gp41 antigen, gp41 glycoprotein antigen, Glycoprotein 41 antigen, Envelope surface glycoprotein gp41 antigen, HIV-1 gp41 antigen, Human Immunodeficiency Virus 1 antigen, HIV1 gp41, Surface protein antigen, SU antigen, HIV 1 gp41 protein, HIV-1 gp41, HIV 1 gp41 antigen