Autoantibodies to the heterogeneous nuclear ribonucleoproteins (hnRNP) have a great role in the diagnosis of rheumatic diseases. hnRNP-A/B proteins are the main autoantigens in Rheumatoid arthritis (RA), systemic lupus erythematosus (SLE), and mixed connective tissue disease (MCTD). Anti-RA33 autoantibodies which target the hnRNP proteins A2, B1, B2 (the 'RA33 complex') help to the diagnosis of RA at early stage. Therefore, anti-hnRNP autoantibodies are not only valuable diagnostic markers but may also allow deep insights into the pathogenetic mechanisms of rheumatic diseases.