Hence, the positivity of anti-RNP is the only biological criterion for mixed connective tissue disease (MCTD) [1], but it is also detected in other autoimmune diseases such as systemic lupus erythematosus (SLE), scleroderma (SSc), Gougerot-Sjögren syndrome (GSS), rheumatoid arthritis (RA) and polymyositis/dermatomyositis (PM/DM) [2]. This evidence concerns the gene RNPC3 and polymyositis.