The nucleocapsid complex is surrounded by a protein shell called capsid to form the viral core. A layer of matrix protein, which is formed outside the capsid, interacts with the envelope (env) which consists of lipid envelope derived from the host cell and viral envelope glycoproteins. Viral glycoproteins are made of two units: a transmembrane portion, which attaches the protein into the lipid bilayer, and a surface portion, which binds to the cellular receptor.