Rabbit Anti-HIV Tat Polyclonal antibody. The transcriptional transactivator (Tat) is a key regulatory protein of HIV. It is expressed early after the virus integrates into the cell, and stimulates the elongation of RNA polymerase II. It binds onto a sequence known as the TAR, or transactivator response element, located at the end of the HIV genetic chain. There, the tat protein helps assemble new copies of HIV. The tat protein-TAR complex speeds up the rate of viral reproduction by about a thousand times. If it is not present, the transcription process frequently stops short, and few functional HIV particles are produced. Tat is an important potential target for antiretrovirals and vaccine development.