RNPC3 and polymyositis: Mixed connective tissue disease (MCTD) is a rare systemic autoimmune disease with an overlapping feature of at least two connective tissue diseases (CTDs), including systemic lupus erythematosus (SLE), systemic sclerosis (SSc), polymyositis (PM), dermatomyositis (DM), and rheumatoid arthritis (RA), with the presence of antibodies against U1-ribonucleoprotein (RNP), especially at high titers [1–4].