The HIV tat protein stands for "trans-activator of transcription," and is a viral regulatory protein. A prevalent form consists of 86 amino acids with a weight of 16 kDa, but can vary by subtype. Tat binds to nascent HIV transcripts and assists in recruiting the transcriptional elongation factor pTEFb to improve the processivity of further viral gene transcription. Targetting tat has been proposed in combatting HIV infections.