The term \'undifferentiated connective tissue disease\' (UCTD) is generally used to describe clinical entities characterised by clinical and serological manifestations of systemic autoimmune diseases but not fulfilling the criteria for defined connective tissue diseases (CTDs). In this narrative review, we summarise the results of a systematic literature research, which was performed as part of the ERN ReCONNET project, aimed at evaluating existing clinical practice guidelines (CPGs) or recommendations. No specific CPG on UCTD were found, potential areas of intervention are absence of a consensus definition of UCTD, need for specific monitoring and therapeutic protocols, stratification of UCTD based on the risk of developing a defined CTD and preventive measure for the future development of a more severe condition. Patients feel uncertainty regarding the name of the disease and feel the need of a better education and understanding of these conditions and its possible changes over time.

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The term \'sclerosing diseases of the skin\' comprises specific dermatological entities, which have fibrotic changes of the skin in common. These diseases mostly manifest in different clinical subtypes according to cutaneous and extracutaneous involvement and can sometimes be difficult to distinguish from each other. The present guideline focuses on characteristic clinical and histopathological features, diagnostic scores and the serum autoantibodies most useful for differential diagnosis. In addition, current strategies in the first- and advanced-line therapy of sclerosing skin diseases are addressed in detail. Part 1 of this guideline provides clinicians with an overview of the diagnosis and treatment of localized scleroderma (morphea), and systemic sclerosis including overlap syndromes of systemic sclerosis with diseases of the rheumatological spectrum.