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The genome of retroviruses contains three major coding regions for virion proteins, gag, pol and env. Gag encompasses information for nonglycosylated viral proteins that form the matrix, the capsid and the nucleoprotein structures. From pol derive reverse transcriptase and integrase, and env codes for the surface glycoproteins of the virion which consist of a transmembrane and a surface domain, linked by disulfide bonds. A viral protease is derived eitherfrom the gagorfrom the pol coding region, depending on the virus. Simple retroviruses contain only this elementary gag, pol, and env coding information. Once integrated, they are able to multiply efficiently, using the cellular transcriptional and replication machineries without intervention of viral transacting factors. Most oncogenic retroviruses belong in this category. Complex retroviruses, on the other hand, encode additional nonstructural proteins from multiply spliced messages. These proteins play important regulatory roles in the life cycle of the virus. They function as transacting factors that, in concert with cellular regulatory proteins, control viral gene expression and function and are essential components in the replication of complex retroviruses. To this category belong the lentiviruses, the spumaviruses and a group of oncogenic retroviruses that includes human T cell leukemia virus (HTLV) and bovine leukosis virus(BLV).