RNPC3 and mixed connective tissue disease: However, as with UCTD, there are several confounding issues that need to be considered: there are at least four different criteria used for classification of MCTD [17]; treatment of MCTD may differ from center to center and may have changed from earlier studies; and the possible protective role of anti-U1 ribonucleoprotein (RNP) autoantibodies, a serological hallmark and criteria for the disease [17].