(1) What are the structures of the CTs of retroviral Env proteins? The answers for this question will give useful information on elucidating a role of the Env CTs in the Env trafficking and/or incorporation in virus-infected cells. (2) What host factor(s) are necessary for the retroviral Env trafficking and/or incorporation into virions? (3) Where and how Env and Gag proteins of retroviruses are recruited to the assembly sites in order to form infectious virus particles?