Antibodies to the Sm antigen represent a highly specific marker for SLE, but only occur in 25 - 35% of patients with the disease. Antibodies to RNP occur in 30 - 40% of patients with SLE, but are not specific for SLE. Antibodies to RNP are present in nearly all patients with Mixed Connective Tissue Disease (MCTD), and are specific for this disorder when all other antibodies are negative. The Sm and RNP antigens are closely associated, part of a protein and small nuclear RNA complex. The Bioplex 2200 ANA screen test system has beads coated with affinity purified Sm, with recombinant antigens RNP-68 and RNP-A, and a separate bead coated with Sm/RNP combined purified antigen.