Two proteins of unknown function are encoded by 3' in-frame extensions of ubiquitin genes. The polypeptides are synthesized as an additional 52 or 76-80 amino acids on the C terminus of ubiquitin, an unusual arrangement conserved in man, yeast and plants (J. Callis and R. Vierstra, personal communication). Although not homologous to each other or to ubiquitin, both extension proteins are highly basic and contain patterns of cysteine and histidine similar to those proposed to form 'zinc fingers'. The longer C-terminal extension protein (CEP80) is 30% lysine and arginine and, when denatured, behaves like a small cationic protein. Its properties after isolation in physiological conditions, however, suggested that CEP80 is part of an RNA-protein complex. Using the antibodies that confirmed the presence of CEP80 in eukaryotic cells, we show here that the protein is located on ribosomes. Immunoblotting of rat 40S subunit proteins specifically identifies CEP80 as ribosomal protein S27a.