HIV-1 life cycle and model of transcription from pre-integrated viral DNA and provirus. Following HIV infection of T cells by specific interaction of viral envelop protein with the CD4 receptor and chemokine co-receptor on T cell surface, the viral RNA genome is reverse transcribed into a full-length double stranded DNA (step 1), and enters the nucleus as a pre-integration complex (step 2). Prior to integration, the non-integrated DNA, in the forms of linear, 1-LTR- or 2-LTR-circles, is active in transcribing all three classes of viral transcripts: the multiply spliced, singly spliced and full-length transcripts (step 3). The multiply spliced, early transcripts such as tat, nef and rev are also translated into products. These early viral factors can enhance T cell activity and promote viral replication process. The non-spliced and singly spliced viral transcripts encoding viral structural proteins are not translated. Following viral integration (step 4), post-integration transcription initiates (step 5). Expression of these transcripts leads to production of progeny virions (step 6).