Synthesized in the early phase of the viral infection, Tat is secreted from T-infected cells circulating in the bloodstream . It is then taken up into uninfected cells where Tat localizes in the nucleus and binds to the long terminal repeat of HIV (LTR) TAR on the viral RNA hairpin. In this way, Tat acts as a regulatory protein of viral gene expression, enhancing the transcription of all HIV-1 genes . This is commonly referred to as the primary role of the Tat protein .