Three African swine fever virus structural proteins of relative molecular weights 150,000, 37,000, and 34,000 (p150, p37, and p34) are derived from precursors with relative molecular weights 220,000, 60,000, and 39,000 (pp220, pp60, and pp39) by proteolytic cleavage after the second Gly residue in the sequence Gly-Gly-Ala/Gly. A search of the National Biomedical Research Foundation Data Bank revealed that several adenovirus proteins, ubiquitin, and an interferon-induced 15-kDa protein are also derived from precursors that are cleaved at the sequence Gly-Gly-X, where X is often an amino acid residue with a hydrophobic side chain. The sequence Gly-Gly-X together with other physical properties of the protein seems to be a recognition sequence for the processing of a variety of viral and cellular proteins.