In hepatocellular carcinoma (HCC), autoantibodies to intracellular antigens are detected in 30-40% of patients. Patients with chronic hepatitis or liver cirrhosis develop HCC, and when this occurs, some patients exhibit autoantibodies of new specificities. It has been suggested that these novel autoantibody responses may be immune system reactions to proteins involved in transformation-associated cellular events. One HCC serum shown to contain antibodies to unidentified cellular antigens was used to immunoscreen a cDNA expression library, and a full length cDNA clone was isolated with an open reading frame encoding 556 amino acids with a predicted molecular mass of 62 kD. The 62-kD protein contained two types of RNA-binding motifs, the consensus sequence RNA-binding domain (CS-RBD) and four hnRNP K homology (KH) domains. This protein, provisionally called p62, has close identity (66-70%) to three other proteins at the amino acid sequence level, and all four proteins may belong to a family having CS-RBD in the NH2-terminal region and four KH domains in the mid-to-COOH-terminal region. The homologous proteins are: KH domain-containing protein overexpressed in cancer (Koc); zipcode binding protein, a protein which binds to a conserved nucleotide element in chicken β-actin mRNA (ZBP1); and a protein which binds to a promoter cis element in Xenopus laevis TFIIIA gene (B3). p62 protein is cytoplasmic in location, and autoantibodies were found in 21% of a cohort of HCC patients. Patients with chronic hepatitis and liver cirrhosis, conditions which are frequent precursors to HCC, were negative for these autoantibodies, suggesting that the immune response might be related to cellular events leading to transformation. However, the possible involvement of p62 autoantigen as a factor in the transformation process remains to be elucidated.