Sequence analysis of multiple isolates of the human immunodeficiency virus type 1 (HIV-1) reveals the existence of a conserved open reading frame, designated T, that partially overlaps the tat,rev, andvpucoding sequences. Here we show that in vitro translation of RNA derived from this region of the viral genome yields a 17 kDa fusion protein, the result of a minus one frameshift event in the overlap between thetatand T open reading frames. It is also shown that messenger RNA species accumulate in HIV-1 infection from which the 17 kDa protein can be made. These observations suggest that ribosomal frameshift events may result in the biosynthesis of viral regulatory as well as viral structural proteins.