Is mixed connective tissue disease (MCTD) a subtype of systemic sclerosis?

In 1972, mixed connective tissue disease (MCTD) was proposed by Sharp et al. as a disease entity characterized by overlapping clinical features of systemic lupus erythematosus, systemic sclerosis (SSc), and polymyositis, as well as high titers of serum anti-U1-RNP antibody. However, the disease concept of MCTD is sometimes far from being sufficiently acknowledged. We, therefore, reported the revised diagnostic criteria for MCTD 2019 by consensus method and evaluation using clinical data of typical and borderline cases of MCTD. The disease entity of MCTD and its difference from SSc can be further emphasized by 1) microvasculopathy detected using nailfold videocapillaroscopy, 2) statistical clustering based on immunophenotypic analysis using flow cytometry, 3) immune cell-specific gene regulation, 4) clinical relevance of anti-SMN complex antibodies.