RNPC3 and mixed connective tissue disease: Mixed connective tissue disorder (MCTD) is a disorder with overlapping symptoms of other autoimmune diseases like Systemic Lupus Erythematosus (SLE), scleroderma, rheumatoid arthritis, polymyositis, and dermatomyositis and is associated with U1-RNP which are antibodies targeted to the U1 small nuclear ribonucleoprotein (RNP) autoantigen.