 Myotonic Dystrophy type I DM1 is the most common form of adult muscular dystrophy, affecting 1 in 8000 individuals. It is an autosomal dominant disorder with multisystemic involvement of multiple organs and tissues, namely brain, heart, endocrine system, eyes and both smooth and skeletal muscles. It results from the CTG expansion of an untranslated region 3 terminal of the DMPK gene which causes a disturbance of the RNA metabolism, in particular defective splicing of various pre-mRNAs such as the muscular chloride channel causing myotonia, the insulin receptor causing diabetes and others. We will carry out an in-depth characterisation of 400 adult DM1 patients identified from local clinical populations across England and through the national DM Registry. Over a two year period we will take measurements 12 months apart to address specific symptoms that cause major quality of life impairment including muscle weakness, myotonia, excessive daytime sleepiness and cognitive impairment. DNA samples will be collected in order to determine the CTG repeat length and serum samples for biomarker identification. We will carry out muscle MRI and sleep studies in a subset of 50 patients. The implemented measures will capitalise on the efforts of previous cohort studies ensuring that all measures are comparable with existing datasets. 