Antibodies against the Sm antigens of the small nuclear ribonucleoproteins (snRNPs) are a very specific marker for the diagnosis of systemic lupus erythematosus (SLE). Antibodies against the Sm proteins are found in 20-30% of SLE patients. Anti-Sm antibodies are observed at a high titer in the active period of SLE and at a low titer in the non-active period. Antibodies directed against Sm proteins as well as DNA found in sera from patients with SLE have been included as one of the American College of Rheumatology classification criteria for SLE. Sm antigen is now known to be consisted of at least nine different polypeptides. Anti-Sm antibodies are predominantly directed against the SmD1 protein of the snRNP complex.