RNP antibody is seen in 95-100% of mixed connective tissue disease and is considered specific for this syndrome if other antibodies are negative; RNP is also present in 20-30% of systemic lupus erythematosus and 15-25% of progressive systemic sclerosis. RNP antigens also contain epitopes that are immunologically indentical to free Smith antigens, therefore, the Smith antibody response must be considered when interpreting RNP results.