mixed connective tissue disease. a combination of scleroderma, myositis, systemic lupus erythematosus, and rheumatoid arthritis, and marked serologically by the presence of antibody against extractable nuclear antigen. mixed connective tissue disease (MCTD) a systemic disease characterized by the combined symptoms of various collagen diseases, such as synovitis, polymyositis, scleroderma, and systemic lupus erythematosus, with a high concentration of antibodies of ribonucleoprotein.