U1 and systemic sclerosis: Mixed connective tissue disease (MCTD) has been defined as an autoimmune disease characterized by high titers of anti-U1 ribonucleoprotein (U1-RNP) antibodies associated with other common characteristics described in other rheumatic diseases such as systemic sclerosis (SSc), polymyositis (PM), rheumatoid arthritis, and systemic lupus erythematosus (SLE), manifesting sequentially in time and being classified in principle as an undifferentiated connective tissue disease [1].