Viewpoint Mixed connective tissue disease: fact or fiction?

The absence of a known causative factor in connective tissue diseases (CTDs) not only hinders correct diagnosis and treatment but spawns a diversity of classification criteria. Although refinements in such criteria have achieved a high sensitivity and specificity, these guidelines are useful more for classification than for diagnosis of individual cases. Furthermore, some patients present clinical manifes-tations of more than a single CTD, whether in concurrent or sequential fashion Such clinical forms with symptoms and signs common to more than one CTD are collectively termed overlap syndromes and their estimated frequency is almost 25%.2 In 1972, Sharp et al3 described a particular overlap syndrome for which they coined the name mixed connective tissue disease (MCTD). It was clinically characterized by