Before we go to the actual vaccine candidates, some knowledge of the structure of the HIV is warranted. HIV, together with the simian, feline, and bovine immunodeficiency viruses (SIV, FIV, and BIV) all belong to the genus lentevirus in the family of retroviridae. These viruses characteristically produce slowly progressive infections, and the replications depends on an active reverse transcriptase enzyme to transform the viral RNA genome into a proviral DNA copy that integrates into the host cell chromosome. This step provides the enormous genetic diversity of HIV isolates in the infected cell, since the reverse transcriptase step is often fraught with errors in replication.