Mixed connective tissue disease (MCTD) refers to one of a number of conditions in which patients have symptoms or signs that suggest a number of different disease entities within the spectrum of the connective tissue diseases (CTDs). The original MCTD, Sharp syndrome, was associated with a specific autoantibody – anti‐U₁‐RNP – occurring in a patient with elements of lupus erythematosus, systemic sclerosis and dermatomyositis, and was recognized as an identity in its own right. Subsequently, other combinations have been described, and may occur with other autoantibody profiles. These have been described as overlap syndromes, or as undifferentiated CTD, or as CTD in evolution because of the range of symptoms and signs and the sometimes changing nature of the condition. There is a tendency for the patient's condition to evolve in the direction of a single CTD over time, although many remain with overlapping features. The condition, however varied, is characterized by a generally good prognosis, and by a good response to therapy, although some patients may be severely affected, and may develop life‐threatening complications despite treatment.

Keywords mixed connective tissue disease, Sharp syndrome, anti‐U₁‐RNP, overlap syndromes, undifferentiated connective tissue disease