Antibodies to the Sm and RNP antigens are diagnostic hallmarks in patients with systemic lupus erythematosus. Both antigens are located on the U1 snRNP (small nuclear ribonucleoprotein particle), a complex of a small RNA and associated proteins that carry the antigenic determinants. This particle also plays an essential role in messenger RNA processing within the cell nucleus. Newer diagnostic techniques are available for detection of anti-Sm and antiRNP antibodies (the latter are now more appropriately called anti-U1 RNP antibodies) and a great deal of information on the molecular biology of snRNPs is currently available.