The HIV-1 virus requires a programmed -1 ribosomal frameshift signal (the HIV-1 Ribosomal Frameshift Signal) for the expression of the Pol gene, which is an example of a cis-acting element of gene regulation. In HIV-1, the "gag" ORF that encodes the 55 kDa Gag protein, the major viral structural protein, is located at the 5' end of the full-length viral mRNA. Translation of the 160 kDa Gag-Pol polyprotein is contingent on a -1 ribosomal frameshift event revealing the "pol" ORF. The "pol" ORF is located 3' to the gag ORF and encodes the Pol polyprotein, which is eventually cleaved into the viral enzymatic proteins (protease, reverse transcriptase, and integrase).