RNPC3 and systemic lupus erythematosus: Mixed connective tissue disease (MCTD) is an autoimmune disorder characterized by high levels of anti-U1 ribonucleoprotein (RNP) antibodies and overlapping clinical features of autoimmune diseases, such as systemic lupus erythematosus (SLE), systemic sclerosis (SSc), and polymyositis (PM).