Antibodies to ribonucleoprotein, RNP, are found often in very high levels in patients with a variety of systemic rheumatic diseases, including SLE, progressive systemic sclerosis and Mixed Connective Tissue Disease, a distinct disease entity with overlapping clinical features of SLE, scleroderma, polymyositis and rheumatoid arthritis. Antibodies to Ribonucleoprotein occur in approximately 50% of patients with lupus erythematosus and in patients with other connective tissue diseases, notably mixed connective tissue disease (MCTD). MCTD is characterized by high levels of RNP antibodies without detectable Sm or double-stranded DNA antibodies. Testing for RNP antibodies is not useful in patients without demonstrable antinuclear antibodies. There are no conditions under which this test should not be done. The test will be of very low yield if the patient has a negative Antinuclear Antibody (ANA).