RNPC3 and polymyositis: Mixed connective tissue disease (MCTD) is a rheumatological condition that presents as an overlap between systemic lupus erythematosus (SLE), rheumatoid arthritis (RA), systemic sclerosis (SSc) and polymyositis, characterized by the presence of anti‐U1 small nuclear ribonucleoprotein (anti‐RNP) antibodies.1