RNPC3 and systemic lupus erythematosus: Mixed connective tissue disease (MCTD) is a chronic, immune-mediated disorder defined by the combined presence of high titer serum anti-ribonucleoprotein (RNP) antibodies and overlapping features of two or more of the systemic rheumatic disorders: systemic sclerosis (SSc), systemic lupus erythematosus (SLE), polymyositis and rheumatoid arthritis (RA) [1].