The virion (particle infectivity) of HIV differs in structure from other retroviruses previously known. It is about 120 nm in diameter and is roughly spherical. Its genome is based physically in two  RNA copies (its sequence is as the corresponding messenger RNA) clothed by proteins, which form the nucleocapsid, and enclosed within a conical capsid, in turn surrounded by a shell of lipid bilayer , stolen first to the plasma membrane of the host cell, but endowed with self proteins. Inside the envelope there are enzymes in the virus, including a reverse transcriptase, an integrase – within the capsid – and a protease. The former is necessary for reverse transcription, DNA synthesis taking the viral RNA as template, and the second for the DNA so produced is integrated into the human genome became provirus.