Sm/RNP Antibody is detected in patients with Mixed Connective Tissue Disease. Antibodies to Sm are highly specific for Systemic Lupus Erythematosus (SLE) and when present are considered a marker antibody. However, these antibodies are found in only 20% of patients with SLE. RNP antibodies are found in 45% of SLE patients but are also observed in numerous other disease states such as Sjögren's syndrome, Scleroderma and Polymyositis. In SLE, RNP antibodies have been associated with a relatively benign disease course with lower incidence of renal and central nervous system involvement.