The genome of HIV is a single-stranded positive small RNA molecule, approximately 9.5 kbs in length and encodes the typical retrovirus proteins Gag further divided into  M(matrix), C(capsid), and N(nucleocapsid); Pol, cleaved into protease, reverse transcriptase, and integrase; and ENV 160 kD glycoprotein, divided into an external gp120  and a transmembrane gp41 subunit that together form    trimeric spikes on the surface of the virion. Furthermore, the HIV genome encodes a variety of nonstructural proteins such as transactivation protein (Tat) splice regulator protein (Rev) and accessory proteins such as Nef, Vpr, and Vpn.