Reverse transcription of the retroviral RNA genome results in the formation of a double-stranded DNA proviral intermediate that is then integrated into the genome of the host cell. The resultant provirus is flanked by two long terminal repeats (LTRs). The 5′ LTR functions as the proviral transcriptional regulatory element while the 3′ LTR mediates efficient polyadenylation of the resultant transcripts (Figure 1). The large majority of retroviruses contain this single LTR promoter element, so that the primary transcript encoded by these retroviruses is identical to the viral RNA genome. The presence of only one promoter element, when combined with constraints on retroviral genome size imposed by the compact nature of retroviral virions, has forced retroviruses to rely primarily on posttranscriptional mechanisms to regulate, and facilitate, the expression of the various viral gene products. Of these mechanisms, the most important is clearly regulated alternative splicing. Genomic organization of ALV and HIV-1. Comparison of the genomic organization seen in ALV, a typical simple retrovirus, with the more complex HIV-1 genome. Viral genes are indicated, as are known major splice donor (D) and acceptor (A) sites. The localization of the Rev response element (RRE) and constitutive transport element (CTE) RNA targets are given. Abbreviations: LTR, long terminal repeat; R, Vpr; U, Vpu. https://s3-euw1-ap-pe-df-pch-content-public-u.s3.eu-west-1.amazonaws.com/9780429503238/debac34e-4517-4ac6-8bd4-ff665833605e/content/fig5_1.tif"/>