The Pol protein is expressed as part of a Gag-Pro-Pol polyprotein, and subsequently released by the viral protease as part of the maturation process. The viral genome packaged into virions is RNA (two copies), and reverse transcription occurs after entry and release of the virion core into the infected cell. In contrast to viruses in the subfamily Spumaretrovirinae, viruses in the subfamily Orthoretrovirinae can differ significantly in terms of genome structure and complexity, host distribution, tissue tropism and disease phenotypes.
Genus demarcation criteria
Genera are currently defined by well-supported phylogenetic clustering, based primarily on alignments of highly conserved domains of reverse-transcriptase and, to a lesser extent, the TM subunit of the Env protein. Viruses in some genera are distinguished by the presence of additional regulatory proteins – for example, the Tax and Rex proteins of deltaretroviruses.